I use my brain, but the software is proprietary.
To the OP: the greatest masters of music all came from a time when apps didn't exist. Nobody who is serious about their musical education uses apps to help their study.
Aw c'cmon, that's just silly. Some of these same greatest masters came from a time when electricity didn't exist, should we therefore only play by candlelight?
You can play by candlelight all you want until the cows come home. J.S. Bach would still eat you for breakfast without any help from his tablet.
Well I don't doubt that, but I also don't doubt that 'ol Johann Sebastian would've made good use of whatever tools helped him do that.
The only tools you need besides a decent instrument are your hands, your eyes, and your ears.
I'm sure they help students learn, but so does a sheet of music.