Zeta Complex

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

The minimum requirements for Zeta Complex are: Windows 7. It runs on Windows only. The recommended specs are in the System requirements section.

No. Zeta Complex is a single-player game.

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

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