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)
Nice post but I think the overview graphic of the linux network queues are not 100% precisely, mostly the links between packets and skb but overall it's great.
FFmpeg can have any numbers of decoders/encoders sometimes you want to use a hardware decode to read and a hardware encoder to write (like the intel), so you can override the default decoders/encoders for each codec.