What is a fast, universal way to launch a particular app (from within any other app)?

Solution 1:

Besides the hardware functionality you mention in the last paragraph of your question, I think the only other solution is to use the NFC support in Shortcuts like this:

Open up the Shortcuts app and tap the Automatics tab in the bottom of the screen. Tap the "Create Personal Automation" button and then choose "NFC". Tap the "Scan" button and hold the NFC tag on to the phone. Then tap "Next" and "Add Action" and choose the app functionality to launch.

Now you can use the tag to trigger the launch at any moment.

NOTE: You need an iPhone XS or later that supports background tag reading.

Solution 2:

There is "Back Tap" accessibility feature in iOS 14 which can be used. It allows to perform various system actions by tapping on the rear of the device.

When the back of iPhone is tapped, an action will be carried out. This includes launching many of the system actions but also launching shortcuts.

Start with creating shortcut to open specific app. Then head to Accessibility > Touch > Back Tap (last choice). Assign double (or triple) tap to shortcut created. Thats it. Now, whenever you double tap on back of your iPhone this app opens.