WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. Features include a plugin architecture...
First, establish what the freelancer needs to do: edit content, transfer files or work on the database. Create a separate application account with only the permissions required. For file access, check whether your hosting service supports a separate SFTP account restricted to the project directory. Avoid sharing client account credentials that also provide access to invoices, domains or other services.
Share access details through a secure channel and agree how long access is needed. Keep a backup before significant changes. When the work is finished, disable temporary accounts, revoke any access keys and change shared passwords that were disclosed.