How do I change the screen going dim so quickly?

Go to Settings/Display & Brightness - Auto-Lock. That is the setting that determines when the screen dims, prior to locking the phone. If you have an iPhone 10 or newer the screen won’t dim if you are looking at it.

Tap the screen. It will go dim shortly before the screen lock settings when the phone is idle, so if you change that setting it will stay on longer.

Then Face ID will not dim the screen as long as it recognizes that it is you who is looking at it.