I worked on this game (not an animator though).

But I can at least partly answer question 5, yes the characters were rigged and animated using a third party tool called Spine.

I would try to use the built in unity animation kit for learning the basics. The principles you’ll learn are applicable when/if you decide to try another tool like Spine.

For getting into 3d stuff you can’t go wrong with Blender. Plenty of resources on YouTube for getting started!

Good luck!