Just compressing the game files. They come as an installer you run. Sometimes they'll come bundled with other goodies/mods/etc. A lot of her repacks also have language packs/4k textures as optional extras.

So the actual cracking/security breaking of the software was already done (fitgirl doesn't crack software herself or at least that's not what she's offering), what Fitgirl does is that she compresses the cracked game down and reduces the file size so that it's cheaper to transfer over the network and store on people's machines. The savings can be upwards of dozens of GBs which translates into money and resources saved.

Unpack a box full of gifts spaced apart loosely for presentation purposes, and then tetris them into a smaller box and ship that instead

Edit: Oh, I just got it -- they teach you that in school huh?