It’s a bit unfair to pull Minecraft as an example for immutability. True, it’s not open source, but it supports mods. So there actually is a way to change this game.

BTW:

Writing a mod is an easier/constrained approach to peak into development.

You learn how parts of the software work, maybe even how they work together. Then you try to modify these parts or their relation, all within the limits set by the creator.

That doesn’t have to involve reading or writing code at all. Maybe your mod will just be some altered numbers in an .ini file. (I’m referring to my 1st mod: a rebalance hack for Duke Nukem 3D. *g*)