But xmanager is not providing the apk, so the project doesn’t do anything illegal.
Also if you root your phone (not illegal for some devices) you can install any app from the store and the apk is just there… on your device, which you can then patch.
It’s also worth checking out Atoms for Peace. It’s Thom York, Flea (from red hot chilli peppers) and the producer of radio head (whatever his name is).