Steam deck can also use the mouse and keyboard UI. And PC can use the gamepad UI. That seems to be the difference you are talking about. It auto switches interface based on what you touched last, so you just need to setup mouse controls on your steam deck, lots of options for how you want to do that. Touchscreen, trackpad, a key combination to activate mouse mode on one track pad and another combination or the same one to turn it back off, plugging a physical mouse in… I’m sure there is more, but either way, one of those should get you the mouse and keyboard UI on steam deck.