WeightWare

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

The minimum requirements for WeightWare are: 250 MB storage. It also runs on macOS. The recommended specs are in the System requirements section.

No. WeightWare is a single-player game.

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

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