for your first question, you will find answer there PHP v7.4 vs v8.x
PHP8 is not available on Debian Bullseye ootb. It would need to be added by a 3rd party repository on Bullseye. Next Debian version Bookworm will support PHP8.2. However, it will not be released before late summer 2023. Best is to wait and to stay on Bullseye / PHP7.4 for moment.
BTW: NC 25 even did not support PHP 8.2 atm 