How can I give access to my developer

To give access to your developer, you have a few options available.

First option: Provide the main Cpanel logins for the relevant account to give full access to the data. Note that you can't generate a separate Cpanel user, only the main one for the relevant account will work.

Second option: Provide access only to the site content (using FTP) and the database.

You can check here how to create a separate FTP account:

You can check here how to provide separate database access