A growing list of useful links across multiple subjects.
https://ianring.com/musictheory/ There are many websites to help understand scales but this is the most authoritative source of scales information you will ever need!
https://www.basicmusictheory.com/ A simple source of keys, scales and chords.
https://muted.io/ Really nice UI experience that outlines a lot of key information.
https://www.hooktheory.com/ Want to know how to play your favorite song? Chances are that this website will have that information.