droidfish
It's Stockfish for Android. You would use it like any other engine.
I wouldn't describe it as something used to improve your game. You can only dial down the strength based on a percentage, leaving you to wonder exactly what rating level your playing at.
You can use it to analyze your game.
I generally eschew having a top line chess engine analyze my games. It far better to self analyze or, ideally, have someone higher rated look at your games.
Humans can understand the level of the player of the game they're looking at and can make comments that are understandable.
Engines usually just spit out the best move and assign a value to it. They have no idea if the move they found would even make sense to a GM, let alone to club player.
I wouldn't describe it as something used to improve your game. You can only dial down the strength based on a percentage, leaving you to wonder exactly what rating level your playing at.
If you dont analyse with engines for improvement, what do you think is the way for improvement? Smoking weeds? LoL.
I use software to train. Chess King standard and I have most of the modules. Although lately I've been thinking about getting some other training material to fill in some gaps.
No matter how good some particular training system is, sometimes it's good to find additional material from a different souce to get a fresh perspective on whatever your having trouble with.