Egg

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

The minimum requirements for Egg are: Windows 10, GeForce 256, Intel Pentium, 256 MB RAM, 50 MB storage. It runs on Windows only. The recommended specs are in the System requirements section.

No. Egg is a single-player game.

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

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