Valve is a relatively small company. They have absolutely fantastic in-house talent (gamescope, for example, is a Valve development and an important innovation of modern SteamOS), but they also have quite a few contractors and projects/teams they support.
(Disclaimer: I worked at a contractor on the Steam Deck for a while, working on the desktop mode.)
(Disclaimer: I worked at a contractor on the Steam Deck for a while, working on the desktop mode.)