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When the users get logged in, the black header of WordPress appears.
That’s the WordPress admin bar. Individual users can disable this from their user profile.
If you want to disable it sitewide for all users, you’ll need to run some PHP code snippet or use a plugin. Please see: https://www.wpbeginner.com/wp-tutorials/how-to-disable-wordpress-admin-bar-for-all-users-except-administrators/
Good luck!