Under What?

Yes. Under What? is free to play on Steam, so there is nothing to buy to start playing.

The minimum requirements for Under What? are: Windows 7, Intel Pentium 4, 512 MB RAM, 100 MB storage. It runs on Windows only. The recommended specs are in the System requirements section.

No. Under What? is a single-player game.

Mostly, yes. Valve rates Under What? as Playable on the Steam Deck: it runs, but may need some manual setup.

Yes. Under What? is the full base game. You need nothing else to play it.