In order to update php versions you have to follow "strict" procedure which is described here:
This applies for vanilla thirty bees and Niara or Community theme. If you use another 3rd party theme (ones inherited from PS) they WILL not work on php8 straight out the box. 99.99% theme has issue with block cart override in the theme itself which leads to Error 500. You have to run your shop on 7.4, fix all notices for such issues then update to 8.0. Again fix all notices and warnings and then update to 8.1, etc.
The update procedure is not vicious its strict and it's made that way because thirty bees dependencies sometimes work on 7.4 but not on 8+ and vice versa.