To designate the main display on macOS with multiple displays connected, follow these steps:
Change Main Display Assignment
- Go to System Settings âž” Displays
- Select your Built-in display icon, then click the drop-down menu next to Use as and select Main
- Alternatively, click the Arrange button, then drag the white menu bar in the Arrangement from your external monitor to your laptop screen icon to designate it as primary
More info from Apple: Displays settings on Mac - Apple Support
Note that with an extended display, you should be seeing the menu bar on both displays. On the active display (the one in which you are currently working), it should be 'normal' and on the other display (without an active app), it should be 'dimmed' but still visible.
If you are not seeing that behavior, then you may need to take additional troubleshooting steps. Have you rebooted your Mac? If not, that's the first step. If that does not help, then see if the problem is present when you boot in safe mode. That process performs some system cleanup and clears caches, and also disables 3rd party extensions that may cause problems.
Start up your Mac in safe mode - Apple Support
If that does not correct the issue, test in a new user account. A newly-created user account will have all settings at default, and if the issue does not occur in that new account then it provides guidance on what may need to be changed to correct the issue.
Add a user or group on Mac - Apple Support
Once the testing is complete, the new account can easily be deleted.
Delete a user or group on Mac – Apple Support
Those steps will help to isolate the problem.
