My "app" is my one and only working ear, plus a knowledge of how to notate music and some music theory, acquired at an early age before all my brain cells started dying off!
I also use the free version of Finale Notepad for notating scores: the playback facility is useful for making sure I've got it right, or for checking that any harmonies are really harmonising. YouTube also helps for finding the version you want of a particular song or tune: 2 screens too, so that you can stop and start the audio, while notating the tune on the other screen. if it's in an odd key, I'll notate it in that key, then use the transpose facility on Finale Notepad to put into a key I can either sing or play.