This may be old hat to others, but I just learned it. yt-download works fine on PC but i hadn’t found a way to do so on Android, particularly since most Invidious instances now have download turned off. I did find, however, that if you use an active invidious instance, such as invidious.tiekoetter.com and use the Android App 1DM+ (probably works on the regular 1DM too, but this app is so good I recommend you support the devs regardless by paying for +) you can access a YT video there and it will download fine, in order to be allowed on PlayStore the app blocks the ability to do so when on YouTube, but never thought about doing it this way until today
Once you select a video and click “Download”, after a few moments, a pop-up like the one in the picture below should appear. You just need to select “Audio” and then “OK”.
If you want, before you press “OK”, you can click the drop-down menu located in the pop-up to choose a different file format or a different bitrate (the higher the bitrate of a file is, the better it will sound).
Happy to help if you have any more questions :)