Not sure about now, but I was a musician with a variety of instruments for about a decade in my youth. In my experience, private instruction was always locally advertised. For instance, I had lessons on oboe, violin and piano. The piano teacher was a friend of my mothers, oboe and violin I had a class at school where the instructor suggested where to get tutors, typically either former students in the local orchestra or students at the local college music program.
To me, an app would have to improve upon this relatively convenient method of finding instructors. Typically these instructors come with the implicit approval of a trusted person as well.