Milk

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

The minimum requirements for Milk are: Windows 7, 128 MB RAM, 100 MB storage. It also runs on macOS. The recommended specs are in the System requirements section.

No. Milk is a single-player game.

Yes. Valve rates Milk as Verified on the Steam Deck: it runs well out of the box.

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