Are there any apps that help with learning musical theory/sheet music?

Recognizing notes is a capacity you basically have to be born with

You can reliably train yourself to recognize intervals (distance between notes) and chord types ("flavor" of multiple notes occuring at once) though and there are trainers for this. Ear training / interval training would be the terms to search for.

yikes those are four completely different skills

music theory is kinda mathy and a fuckload of jargon

sheet music, assuming you mean western notation, is a literacy thing and probably requires a bunch of shitty memorization and looking at transcriptions of songs you already know. plus the lines and spaces mean different shit depending on what instrument it's for, some use two, some use a completely different system. I used to be able to read music and i managed to lose that ability in highschool despite singing and playing an instrument the whole time.

recognizing a pitch is a fuck and I don't think my public school music education did anything for this, there's for sure going to be apps for this that play a random tone or combination of tones and then have you type in or guess what it was.

and of course finding a specific note on an instrument is going to be completely different depending on the instrument, oh and by dumbass western convention the same frequency on two instruments will be notated differently because fuck you.

not to discourage you but from how you've phrased the question you probably need some basic instruction from a person to build a foundation before you go chasing what amounts to coursework.

I’ve heard good things about Chord Crushers, but I haven’t tried it