I don't know about the latest Stream Deck models, but for previous models (at least the ones I have) you might be surprised that behind the scenes is just one display! See the teardown (album not mine): https://imgur.com/gallery/elgato-stream-deck-what-is-inside-...

The buttons are entirely virtual and the onboard controller ensures they are properly positioned, so you don't have to have each button carry it's own screen.