Nothing?
Best iOS app focussing on Strategy/Middlegame?

Not sure about apps — you might want to search the App Store— but a good introductory book on strategy is Simple Chess by Michael Stean. It’s not as simple as the title suggests, but it will point you in the right direction for looking at the strategical aspects of the game.
https://www.chess.com/article/view/how-to-play-positional-chess
https://www.chess.com/article/view/test-your-positional-chess
https://www.chess.com/article/view/do-you-really-understand-positional-chess
https://www.chess.com/article/view/get-ready-to-test-your-positional-chess-again
https://www.chess.com/article/view/can-you-pass-this-positional-chess-test
https://www.chess.com/article/view/how-to-calculate-long-term-advantages-in-chess
https://www.chess.com/article/view/learning-basic-pawn-structures
https://www.chess.com/article/view/learning-pawn-structure-for-chess-players-under-2000
https://www.chess.com/article/view/your-pawn-structure-is-your-friend
https://www.chess.com/article/view/more-pawn-structures
I‘m struggling heavily during the transition from the opening- to the middlegame. It‘s like I‘m lost and no movement seems to make any sense.
Are there any good iOS apps for this problem? Tactics are not the issue here... but strategy is. Chess.com is a hell of a mess full of nugget-infos... can’t get used to it. The lessons... well too short, these are just an overlook on all topics.
If you as well could mention one superb book which provides tools to utilize/ideas for my strategy game.
I feel like blind/hitting a wall... no idea what to do.
Thanks folks!!
Cabal
Additional question: Is chess.coms Chess Mentor/Lessons really that extremely thin?