YoutubeDL (not to be confused with youtube-dl), is an Android app written in C# using Xamarin that allows you to easily download Youtube videos as MP3 files. This is achieved using YoutubeExplode. The entire source code is available on Github. Released APK files can also be found there.
Because Google does not appreciate the downloading of Youtube videos, this app will not be released on Google Play.
Here’s how it works: After installation, open the app once to grant permission to read and write to storage. Then, open the Youtube app, pick a good song, and share it to YoutubeDL. The app should immediately return you to Youtube and start the download. Meanwhile, you can continue doing what you were doing, including downloading additional videos.