This tweak makes Spotify think that you have a Premium subscription, granting free listening, just like Spotilife.
Several months ago, Spotilife, the only tweak to get Spotify Premium, stopped working on new Spotify versions. I decompiled Spotilife, reverse-engineered Spotify, intercepted requests, etc., and created this tweak.
Upon login, Spotify fetches user data, including active subscription, and caches it in the offline.bnk file in the /Library/Application Support/PersistentCache directory. It uses its proprietary binary format to store data, incorporating a length byte before each value, among other conventions. Certain keys, such as player-license, financial-product, and type, determines the user abilities.
The tweak patches this file while initializing; Spotify loads it and assumes you have Premium. To be honest, it doesn't really patch due to challenges with dynamic length and varied bytes. Ideally, there should be a parser capable of deserializing and serializing such format. However, for now, the tweak simply extracts the username from the current offline.bnk file and inserts it into premiumblank.bnk (a file containing all premium values preset), replacing offline.bnk.
Tweak also changes query parameters trackRows and video in HTTP requests to true, so Spotify loads videos and not just track names at the artist page. Sorry if the code seems cringe; the main focus is on the concept. It can stop working just like Spotilife, but so far, it works on the latest Spotify 8.9.## (Spotilife also patches offline.bnk, but it changes obscure bytes that do nothing on new versions). Spotify reloads user data from time to time (and on changing network, for example), so if Premium stops working, simply restart the app.
There is no offline or very high quality (similar to Spotilife) because these features are server-sided (i.e., you can't get very high quality tracks from the server without a subscription). In theory, it might be possible to implement offline mode locally, but not in this tweak.
To open Spotify links in sideloaded app, use OpenSpotifySafariExtension.