I’ve been using Deemix for several years, but it’s not maintained anymore and it finally stopped working :<
But the way it worked was you paid for a deezer.com subscription and gave the login arl token to the deemix app. Then the app could download the music files using your account.
Is there anything like that these days?
You could try Spotube. It’s sort of a front end for your Spotify account and can download songs. Can be buggy at times but it works.