Hi Henry,
Well, I think that is server dependent to some extent, but this should contain all the info: https://wordpress.org/support/article/changing-file-permissions/
For ownerships, ususally the www-data user needs to be the owner of the files, but that can be different. Depending on your server and path, usually a similar command should do:
sudo chown -R www-data:www-data /var/www
Please be very careful running SSH commands, always make sure to have a back-up.