Hello, so I am new here and I hope this is in the right place, honestly couldn't find anywhere else for it.

I play a game from 1999 called heroes of might and magic 3. They have recently (few years ago) released a patch for it to put it in HD and make several other changes to the game like polishing some elements of gameplay. Anyway, I am really curious how they do this. Did they get the actual source code somehow and mod it that way? Everything is integrated flawlessly to the style of t he game. They have an area for plug-ins in the mod now and I kind of wanted to try to mod it a bit myself. I have no idea how to begin. Wouldn't it be illegal to get the source code? How do they make these mods?

SO how is it done, son?