Prefabs can be made using the prefab editor included with the game, which is one of the option you get when you first start it up--New game, Join a server, what-have-you. Think it's about the 5th option from the top. They're built with the same blocks as you use in game, plus others not craftable by players in the vanilla game. Physics seem to be off in the prefab editor by default, meaning you can build things that ignore SI (structural integrity) rules of the game until they actually get put in the game... where they may fall apart. The tile system introduced in A20 uses a very small set of POI footprints, so you'll want you building to fit one of those, I'd imagine.
The game is made in Unity, but not the very latest version available on the Unity website. You'll probably want to install the Unity Hub if you plan on running more than one version of Unity. I keep 2 version on the Hub currently--the one used for A19, and the one for A20. Not at home at the moment, so can't give you the exact build, but I think it was something like 2019.1 and 2020.1, respectively.
If you're planning on doing modelling, you'll want Blender, though you can also bring models and animations from Mixamo, MakeHuman, SketchUp, etc into Unity.
For image editing, GIMP comes in handy. Image type doesn't seem as important as image proportions, as Unity can compress textures that are powers of 2 (so, 256x1024 pixels or things like that).
That's just the tip of the iceberg, but I hope it's enough to get started.
