How to learn c# in unity fast?

Answer 1

If I were you I would complete the C# course in Sololearn, and then look at some YouTube tutorials on how to apply your knowledge to development in the game engine.

Answer 2

You can buy courses on Udemy, or PluralSight, there are much excersise on Microsoft Docs for C# and more.
For unity, you can go to the official site of unity documentation , is very good https://docs.unity3d.com/Manual/index.html

But as Edijs said, complete the C# course here then go to look for more docs