A few third party keyboards have a keyboard key that will raise and dismiss the software keyboard. Apple’s own keyboards do not, The only way to get them to release the software keyboard is to disconnect. If it’s Bluetooth keyboard, that means turning it off, or disconnecting Bluetooth. For Apple’s smart keyboard you can fold back the keyboard so that it no longer props up the iPad, or remove it altogether.
Something I just noticed. If you use the emoji keyboard and a third party software keyboard it’s possible to keep the thrid party keyboard up, if you select it right after pulling up the emoji keyboard. Weird.
Use the world key to pull up the emoji keyboard, then tap it again and select the third party keyboard. The onscreen keyboard will go away as soon as you select a system keyboard.
It takes a bit of practice.
Hmm, maybe I should clean my keyboard.