What are the softwares we should master for developing 2D games in Unity?

Answer 1

Basically you will just need to know Unity itself in order to put your game together. However you will also need some way of creating your 2D graphics. A couple of free programs which could work for this would be Krita or GIMP. Photoshop works great if you have it.
For writing your C# code in order to add scripts to your games, you will also need some kind of text editor - Visual Studio Code works pretty well for me.