If you buy a Steam Deck and just use it as a handheld console and never select "reboot to desktop mode" it will act just like a closed console. The exceptions compared to something like a Switch:
- For some games (usually those oriented around keyboard and mouse) you need to go and select one of the community control configurations, and maybe tweak it a bit. For example, I needed to do this with FTL to make it easily playable
- Occasionally (and I've basically had to do this once, in my 2+ years with a Steam Deck) you need to go and select a different Proton version to make it work. ProtonDB tells you what to do
This is all rare though. The vast majority of games have a control setup for using a controller, and they definitely do if they've ever been released on console. And they will Just Work.
- For some games (usually those oriented around keyboard and mouse) you need to go and select one of the community control configurations, and maybe tweak it a bit. For example, I needed to do this with FTL to make it easily playable
- Occasionally (and I've basically had to do this once, in my 2+ years with a Steam Deck) you need to go and select a different Proton version to make it work. ProtonDB tells you what to do
This is all rare though. The vast majority of games have a control setup for using a controller, and they definitely do if they've ever been released on console. And they will Just Work.