BTW, this is how birds learn their calls! The only app or teacher or tuning device you will ever really need are your own ears. We musicians develop technique by training ourselves to listen and sing using our listening as the guide— this is how we improve. No person or device can do this for you— this is what singing is all about just like logic is what programming is all about. There is no shortcutting this— it is myelinating those nerve fibers through practice and listening and developing an opinion of your own about it. The mindset is not about getting a badge from a qualified judge— that judge is you and only you. The mindset is about adventure and experimentation and creativity and expression of style.