Unusable Macbook pro screen
It could be as simple as a worn or partly disconnected ribbon cable, but the centre section makes me think it's physical screen damage. Without hands-on it's hard to say.
Your choices are… re-seat or change the cable &amp; see if it improves, or take it to a professional. The cables are very fragile and this new a Mac might be covered if it’s failing in the terms of coverage.
Your test of an external monitor eliminates the GPU being at issue. One further test would be take a screenshot &amp; view it on the external monitor.