How do I get my apps to fill full screen on my ipad

Some Apps are principally designed for iPhone - and will not fill the iPad screen.

Are you using Stage Manager, or some other multitasking mode? Stage Manager can also have impact upon how Apps are proportioned on-screen.

If you can provide more information, of perhaps post a screenshot (redacted to remove any personal or sensitive information), contributors may be better able to offer appropriate guidance.

Take a screenshot on your iPad - Apple Support

You post your screenshot using the image button within the message editing window: