How do I know which linux user Wordpress uses for plugin installation

Well it's not about /wp-content file permissions, so just leave the WordPress directory directory /wp-content permissions as 0755.

The fix that is required here has to be initiated within apache2.conf file; there we need to change "AllowOverride" directive for our /www/html/ directory from "AllowOverride None" to "AllowOverride All"

Follow the complete procedure here How to deal with GCP WordPress error "This page isn’t working example.com is currently unable to handle this request. HTTP ERROR 500

With this we tell a web server to allow a use of .htaccess file.

The Wordpress user is also the web server user, so this should work:

ps -ef | grep http

or

ps -ef | grep apache

On your production server, in your WordPress index.php file, at the top, you can temporarily put echo(exec("whoami"));die();

Then browse to your WordPress site and see what user was running. On Ubuntu, mine was www-data.

This was useful for me for: Can I install/update WordPress plugins without providing FTP access?

So it clearly says that a group has no W access.

You can do: chmod -R g+w /your/plugins/dir

And then, nginx group will be able to write there.