That's a really nice introduction. One newbie question: how is this influenced by DRM on the browser? Is it all the same plus some security on top or do videos with DRM use a proprietary codecs and packaging?
Decryption keys are then exchanged using one of the common proprietary DRM protocols, usually Widevine (Google), Playready (Microsoft), or Fairplay (Apple). The CDM (Content Decryption Module) in the browser is then passed the decryption key, so the browser. Can decrypt the content for playback.
I think the DRM part is made during the packaging phase the https://howvideo.works/#packaging (where they need to signalize the media has drm and also encrypt the media)