RYB

Yes. RYB is free to play on Steam, so there is nothing to buy to start playing.

The minimum requirements for RYB are: Windows 7, Intel Atom, 2 GB RAM, 200 MB storage. It also runs on macOS. The recommended specs are in the System requirements section.

No. RYB is a single-player game.

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

Yes. RYB is the full base game. You need nothing else to play it.