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This message doesn't say much more than "something on your site broke because PHP 8 is being anal retentive about variable types". It doesn't tell us what broken.

So, as a minimum, if you can downgrade your site to PHP 7.4 it would let you access it. If you're happy with that solution stop here.

If you want to find out what broke you need to edit your configuration.php file and set

public $debug = true;

in the corresponding line of that file. You will now get a more detailed message which will tell you which file and line threw that error as well as the call stack (how we got here). If you can't figure it out, copy and paste that information to your next reply.

As for main.php *cough cough* you found it out yourself :D For those who didn't, it's here: https://www.akeeba.com/documentation/admin-tools-joomla/atwafissues.html You now rename the file services/provider.php file.

You need to ask your host.

On hosts using mod_php (the Apache module, very archaic way of doing things) you can use .htaccess code to set some PHP limits, including the post_max_size and upload_max_filesize. You can also create php.ini files in the site's web root and the administrator folder, unless it's been explicitly disabled by the host.

On hosts using CGI, FastCGI or PHP-FPM you can typically create a .user.ini file in your site's web root. Just note that your changes there apply after ~5 minutes (it's cached by PHP) and only if it's not disabled by the host.

That said, most hosts have a subset of the PHP configuration available for editing through the hosting interface. cPanel-based hosts typically have cPanel's EasyPHP which does offer a modicum of configuration options, definitely the two aforementioned options.

I have yet to see a commercial host which does not allow any change whatsoever. That's the kind of things we'd do on a server we are explicitly locking down. This is diametrically opposite to commercial hosting :)

Go to the backend of your site, Components, Admin Tools, Web Application Firewall, Configure WAF, click on Save & Close.