You would need to ask your hosting company where the directory is for your site. This is not something we are able to answer, as we dont host your site.
You are correct in that you need to be on PHP 8, and that it will take your site down when you move. What you need to do is
Switch off your site
Create a full backup of your site
Switch to PHP 8 (yes, your site will be unoperable at that point)
Run the requirement checker to ensure things check out
Upload a fresh set of files from your client area
Run the upgrader from /admin/upgrade
This does not take into account any 3rd party items you have, and you should ensure those are going to be compatible with the latest release before you proceed.