PHP versions and extensions, Node.js and package managers.
Yes. Each version gets its own PHP-FPM service, and every site chooses its own, so an old application can stay on 8.3 while a new one uses 8.4. You install versions from the Runtimes tab. See Choose and switch PHP versions.
The command line uses the CLI default, and each site uses its own PHP-FPM version. That is normal. Ask for a specific version with a command such as php8.3 -v. See Choose and switch PHP versions.
Switch it on for the right PHP version on the Runtimes tab. If the error persists, the site is probably on another version than the one you changed, so check the version's Sites column. See PHP extensions and php.ini settings.
Something else in the chain is smaller: the application's own limit, or client_max_body_size in the site's Nginx file, which allows 64 MB by default. PHP's upload and post limits must also be large enough. See PHP extensions and php.ini settings.
Yes. You can pin a version per site, and a pinned site ignores the server-wide change. pnpm and Yarn are enabled through Corepack with one button, and Bun is installed from the same Node.js card. See Node.js versions and package managers.
Add the credential on the server's Packages tab, or on the site, for the same Linux user the site runs as. The host must match exactly, and a site credential beats a server one. See Composer and global tools.
That is what the setting does: PHP stops checking files for changes. Reload PHP-FPM from the version's menu, or let your deploy script do it. See PHP extensions and php.ini settings.