The WordPress Codex describes the different user roles baked into the core software (https://codex.wordpress.org). Just search Roles.

If you want to see for yourself what each role is capable of accessing just establish new user accounts at those user role levels then sign out of your administrator account and sign back in as each new user. You will see what that role access looks like. then you can sign back in with your admin account and delete them.

Yes, the super-admin role is for a multi-site installation - that role has admin access to all of the sites in the installation, while each separate site has an admin role that can modify only that site - within the parameters set by the super-admin.

A WordPress developer will need at a minimum an admin access account to the WordPress installation in order to make coding changes to the theme that you specify. cPanel access is not totally necessary if you set up an ftp account within your hosting that the developer can use to access files. When you have terminated your agreement you should delete the ftp account and the WordPress admin account. You would probably also want to change your own admin account password just in case.