If you play a string instrument and want to learn the notes on each strings, the notes on music staff, etc. the FretTrainer APP is good.
There are 6 instruments available to practice on: Guitar, Bass, 5-String Bass, Mandolin, Ukulele, Banjo.
https://www.theguitarjournal.com/fret-trainer-app-now-on-i…/
It is free, but has ads.