How to install php8.0-mbstring for php8.0? It keeps installing to php7.2

I am trying to install Laravel. I am on Ubuntu 18.04.5 LTS, have php 8.0.2, and running Apache2. My issue is that I can't install Laravel as it keeps giving the error that I don't have the 'mbstring' extension.

When I do the commands

sudo apt update

sudo apt install php-mbstring

they get installed to /etc/php/7.2/mods-available. Laravel is looking for the extensions in /etc/php/8.0/. I have also tried installing the newest version of mbstring:

sudo apt install php8.0-mbstring

But get the error

Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package php8.0-mbstring
E: Couldn't find any package by glob 'php8.0-mbstring'
E: Couldn't find any package by regex 'php8.0-mbstring'

Can someone help me figure this out? Why does mbstring not get installed to php8.0, and why can't I install mbstring for 8.0? I have googled online and found guides for installing mbstring for 8.0 and this specific command, but I can't see that extension.

Figured it out thanks to Michal Przybylowicz's comment.

The website's repository: https://launchpad.net/~ondrej/+archive/ubuntu/php needs to be added to Ubuntu's package system. The site lists the commands to do:

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update

I also did:

sudo add-apt-repository ppa:ondrej/apache2

since I run apache2. After that Laravel said that I need the 'dom' extension as well. That was solved by:

sudo apt install php8.0-dom

After that Laravel finally installed! Thanks Michal!