This webpage is intended to help developers quickly generate user tokens from their developer tokens from the Apple Music Auth flow for debugging purposes. This is generally handled by the SDKs that Apple provides.
Go to https://antonino-u.github.io/apple-music-token-helper/ and input the value of the JWT that you obtained from here (or anywhere else for that matter), paste it into the web page, and then generate a user token. Check the console of the web page in your web inspector if you want to play around with the API some more/if you're curious.