This app was mentioned in 3 comments, with an average of 2.00 upvotes
Let me tell you, there are TONS of ways to learn basically any instrument these days!
There's yousician for piano, guitar, bass and ukulele.
If you want recommendations for instruments let me know, you can get a decent guitar for pretty cheap these days and it doesn't take much room. And digital pianos have gotten really good, I just recently bought a Roland FP-30 (which is their cheapest) and I'm just loving it! But moving on!
There are tons of apps to help you practice musical theory, notes, rhythm and ear-training.
For musical theory, I mainly use my books on it, but here's a few that works:
Music Theory Helper: I've tried this one and it's great for the basics but I needed something a bit more in-depth.
Music Theory: have not tried this one but it looks good!
Perfect Ear Trainer: This right here is one of my best friends, it's got theory pages, drill, rhythm and pitch exercises. The best part about is that it goes through all the major things you need to understand music. Chords, intervals and scales are all there and you get to learn how to train every part of it, including absolute pitch which is super hard!
Moving on to ear training but first I wanna just point out that Perfect Ear Trainer just above does this as well even though it is not really its strong suite.
Functional Ear Trainer: Sounds the same, sooooo different. This is what I use to help me with relative pitch identification. It is super straightforward! It plays you a cadence (that is a progression the four important chords of the key) to establish the key, and then a note belonging to that scale. Your job is to pick out which note in the key was played, and if you do it right, it will resolve down to the root. It is infuriating but really easy to practice. So do a little each day!
Complete Ear Trainer: Have not tried this one, but it was high up on Google Play and it says that it's built like a video game if that helps?
For notes, it has become so much easier to practice this using simple quiz-apps, that goes for fret boards as well. These are all pretty straightforward so I'm just gonna list a few that looks good.
SolfaRead: Guitarhero but with less fake crowds.
Note Teacher: It's so-so, but you can set the key to basically anything and it has the bas-staff AND connects to a midi-keyboard if you can plug it in! You can also set the range and speed which is really nice if you're new.
Note Trainer: Supposedly 'fun' game about reading notes, I kinda doubt it but if it works it works. It's all basically memory so anything that floats your boat!
you can use digital piano and digital guitar app on android/apple
Digital Guitar App
https://play.google.com/store/apps/details?id=br.com.rodrigokolb.realguitar&hl=en&gl=US