The confusion here is that this has been explained on quite a few occasions. We are trying to assist, but there is only so far we can go to assist on a self-hosted environment that you're not sure how to run correctly, unfortunately. Our advice at this stage is to go with a more managed environment (our cloud product) or employ someone to manage this part of the site on your behalf, as you are clearly struggling with this.
In answer to your question, what you need to understand is that you are on a very old version, so have missed the transitional steps between where you are now, and where you need to be. By this, I mean there were versions between which could use both php7 and php8, where you could upgrade, then switch. This is no longer the case and we no longer support PHP 7 at all.
So, with the above statement, this means you can not use the automatic upgrader in the admin CP, and your site will indeed crash/break once you update the server to PHP 8.
The process you need to follow is below
Do a full backup of your site in its entirety.
Switch your server to PHP 8 (Yes, your site will break. This is expected)
Use the requirement checker to ensure you have all the minimum requirements on your PHP installation. You can find this at the following location - https://invisioncommunity.com/files/file/7046-get-ready-for-ips-40/
Upload a fresh set of files from your client area
Visit /admin/upgrade in your browser to visit the upgrader directly. Log in and run the upgrader
The problem you will inevitably have here, are 3rd party items you have installed there. This is the issue you have had a few times before. If you have 3rd party items that are not compatible with PHP 8 and the latest release, it is likely it will either break your site once its upgraded, or break the upgrader itself. You need to check these with the developers of those apps and plugins to see what is the best way forward. Our advice if unable to manage these would be to remove as many of these as possible.
Again, I really cant emphasize enough how much I would advise you get assistance from someone to manage your site on this side for you if you are struggling with it.