Answer 1

It just depends upon the game. Unity is really good about it's cross-platform abilities, so you could use Unity and C# to create games on all of those platforms easily and effectively.
http://unity3d.com/learn/
https://www.sololearn.com/Course/CSharp/
It's one of the main engines used in the gaming industry, so you'll been in good hands and have access to plenty of resources & community. Unreal Engine is really good as well, but if you're just getting started, I think Unity will be a little easier to ease into. It's worth checking out UE later on though, it's an amazing engine too.
https://www.sololearn.com/Course/CPlusPlus/
https://www.unrealengine.com