Tree

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

The minimum requirements for Tree are: Windows 7, GeForce 7800, Intel Core 2 Duo, 2 GB RAM, 120 MB storage. It runs on Windows only. The recommended specs are in the System requirements section.

No. Tree is a single-player game.

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

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