ethanuppal.github.io/transcribe
Transcribe is a web app that lets you slow down and loop sections of music. It is entirely free to use, and although the website does not store any information about users, you can export and import your progress to pick up where you leave off.
I personally use this app very often in my music studies and find it to be very useful, but if there is anything you feel would improve the app, feel free to file an issue in this repository.
You can find an overview of how to use the app on the help page.