Using virtual keyboards with external keyboard, how?

My use case: use colemak mod dh with my external keyboard.

iphone 16 pro max.

If I can get to use apps like keybuild (or similar ones) with my external keyboard solves the problem.

But I am open to any suggestion to use colemak mod dh with an external keyboard.

Can't figure out how to use for example keybuild which is an external keyboard with an external keyboard. When external keyboard gets connected via bluetooth, another keyboard gets picked up.