How to open all the Bootstrap 5 accordions simultaneously without closing the ones that were already open

Solution 1:

Hopefully, starting to solve part of your request? All the accordian-items will open and close simultaneously when clicking any accordian-item.

<html>
   <head>
      <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-MrcW6ZMFYlzcLA8Nl+NtUVF0sA7MsXsP1UyJoMp4YLEuNSfAP+JcXn/tWtIaxVXM" crossorigin="anonymous"></script>
      <link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous">
   </head>
   <body>
      <div class="accordion" id="accordionPanelsStayOpenExample">
         <div class="accordion-item">
            <h2 class="accordion-header" id="panelsStayOpen-headingOne">
               <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target=".panelsStayOpen">
               Accordion Item #1
               </button>
            </h2>
            <div id="" class="panelsStayOpen accordion-collapse collapse">
               <div class="accordion-body">
                  <strong>This is the second item's accordion body.</strong> It is hidden by default, until the collapse plugin adds the appropriate classes that we use to style each element. These classes control the overall appearance, as well as the showing and hiding via CSS transitions. You can modify any of this with custom CSS or overriding our default variables. It's also worth noting that just about any HTML can go within the <code>.accordion-body</code>, though the transition does limit overflow.
               </div>
            </div>
         </div>
         <div class="accordion-item">
            <h2 class="accordion-header" id="panelsStayOpen-headingTwo">
               <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target=".panelsStayOpen">
               Accordion Item #2
               </button>
            </h2>
            <div id="" class="panelsStayOpen accordion-collapse collapse">
               <div class="accordion-body">
                  <strong>This is the second item's accordion body.</strong> It is hidden by default, until the collapse plugin adds the appropriate classes that we use to style each element. These classes control the overall appearance, as well as the showing and hiding via CSS transitions. You can modify any of this with custom CSS or overriding our default variables. It's also worth noting that just about any HTML can go within the <code>.accordion-body</code>, though the transition does limit overflow.
               </div>
            </div>
         </div>
         <div class="accordion-item">
            <h2 class="accordion-header" id="panelsStayOpen-headingThree">
               <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target=".panelsStayOpen">
               Accordion Item #3
               </button>
            </h2>
            <div id="" class="panelsStayOpen accordion-collapse collapse">
               <div class="accordion-body">
                  <strong>This is the third item's accordion body.</strong> It is hidden by default, until the collapse plugin adds the appropriate classes that we use to style each element. These classes control the overall appearance, as well as the showing and hiding via CSS transitions. You can modify any of this with custom CSS or overriding our default variables. It's also worth noting that just about any HTML can go within the <code>.accordion-body</code>, though the transition does limit overflow.
               </div>
            </div>
         </div>
      </div>
   </body>
</html>

Solution 2:

I don't think Bootstrap has anything by default for your test case. What you want is to detect the show or hide events on the collapse, and then manually control every accordion. I took the basic example from their documentation, and modified it. Do you want something like this?

const accordion = document.getElementById('accordionExample'),
        accordionCollapses = Array.from(document.querySelectorAll('.accordion-collapse'));

accordionCollapses.forEach((accordionCollapse) => {
  accordionCollapse.addEventListener('show.bs.collapse', (event) => {
    accordionCollapses.forEach((accordionCollapse) => {
      accordionCollapse.className = 'accordion-collapse collapsing';            
      accordionCollapse.style.height = '100%';
      setTimeout(() => accordionCollapse.className = 'accordion-collapse collapse show', 350);
      accordionCollapse.parentElement.querySelector('.accordion-button').className = 'accordion-button';
    });
  });

  accordionCollapse.addEventListener('hide.bs.collapse', (event) => {
    accordionCollapses.forEach((accordionCollapse) => {
      accordionCollapse.className = 'accordion-collapse collapsing';
      accordionCollapse.className = 'accordion-collapse collapse';
      accordionCollapse.parentElement.querySelector('.accordion-button').className = 'accordion-button collapsed';
    });
  });
});
<!DOCTYPE html>
<html lang="en">

  <head>
    <meta charset="UTF-8">
    <title>Document</title>
    <link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous">
  </head>

  <body>
    <div class="accordion" id="accordionExample">
      <div class="accordion-item">
        <h2 class="accordion-header" id="headingOne">
          <button class="accordion-button" type="button" data-bs-toggle="collapse" data-bs-target="#collapseOne" aria-expanded="true" aria-controls="collapseOne">
            Accordion Item #1
          </button>
        </h2>
        <div id="collapseOne" class="accordion-collapse collapse show" aria-labelledby="headingOne" data-bs-parent="#accordionExample">
          <div class="accordion-body">
            Less content
          </div>
        </div>
      </div>
      <div class="accordion-item">
        <h2 class="accordion-header" id="headingTwo">
          <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#collapseTwo" aria-expanded="false" aria-controls="collapseTwo">
            Accordion Item #2
          </button>
        </h2>
        <div id="collapseTwo" class="accordion-collapse collapse" aria-labelledby="headingTwo" data-bs-parent="#accordionExample">
          <div class="accordion-body">
            <strong>This is the second item's accordion body.</strong> It is hidden by default, until the collapse plugin adds the appropriate classes that we use to style each element. These classes control the overall appearance, as well as the showing and hiding via CSS transitions. You can modify any of this with custom CSS or overriding our default variables. It's also worth noting that just about any HTML can go within the <code>.accordion-body</code>, though the transition does limit overflow. <strong>This is the second item's accordion body.</strong> It is hidden by default, until the collapse plugin adds the appropriate classes that we use to style each element. These classes control the overall appearance, as well as the showing and hiding via CSS transitions. You can modify any of this with custom CSS or overriding our default variables. It's also worth noting that just about any HTML can go within the <code>.accordion-body</code>, though the transition does limit overflow. <strong>This is the second item's accordion body.</strong> It is hidden by default, until the collapse plugin adds the appropriate classes that we use to style each element. These classes control the overall appearance, as well as the showing and hiding via CSS transitions. You can modify any of this with custom CSS or overriding our default variables. It's also worth noting that just about any HTML can go within the <code>.accordion-body</code>, though the transition does limit overflow.            
          </div>
        </div>
      </div>
      <div class="accordion-item">
        <h2 class="accordion-header" id="headingThree">
          <button class="accordion-button collapsed" type="button" data-bs-toggle="collapse" data-bs-target="#collapseThree" aria-expanded="false" aria-controls="collapseThree">
            Accordion Item #3
          </button>
        </h2>
        <div id="collapseThree" class="accordion-collapse collapse" aria-labelledby="headingThree" data-bs-parent="#accordionExample">
          <div class="accordion-body">
            <strong>This is the third item's accordion body.</strong> It is hidden by default, until the collapse plugin adds the appropriate classes that we use to style each element. These classes control the overall appearance, as well as the showing and hiding via CSS transitions. You can modify any of this with custom CSS or overriding our default variables. It's also worth noting that just about any HTML can go within the <code>.accordion-body</code>, though the transition does limit overflow.
          </div>
        </div>
      </div>
    </div>
    <script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js" integrity="sha384-MrcW6ZMFYlzcLA8Nl+NtUVF0sA7MsXsP1UyJoMp4YLEuNSfAP+JcXn/tWtIaxVXM" crossorigin="anonymous"></script>
  </body>

</html>

JSFiddle, if you need.