How can application like "etude" or "autoflip" work?

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Android

I am going on a project, something like the two apps that I mentioned in the title. And I can seldom found how such application work with the search engine, so here I come. Thanks in advance. :)

I am sorry that I did't specify the question.
"etude" or "auto flip" are the applications for one who plays piano with no need to flip the music sheet manually.

Here are some questions that I wish you can give me a help;
1. How to display the music sheet, with which format?
2. How can I compare the information of music sheet with the real-time voice, and find out the pinpoint to flip the music sheet?
3. How can the voice with multiple notes at the same time be recognized? This question is optional for now. I think I should finish recognizing one note in a time first.

And, my chaotic thought will be given in the following, if you don't want you thought to be blocked, you should not continue reading it.
(...Think...)

(...Think once)

(...Think twice)

Ok, my chaotic thought now is that using MIDI to format the music sheet, and then convert the real-time voice to some format, like frequency space, whatever; and then compare it with the MIDI, thought I don't know how, this is the problem. And I think it should have a better way to do. Thanks for watching all of this!! :P