Identify (by IP or traffic shape) video traffic, then throttle the stream to the desired bitrate. The viewer's player will usually silently adapt.
Netflix should have it own set of AS. And after that it's quite simple, routers can do throttle for decades.
If it is an HLS or DASH type of video stream, they could just query the playlist for a smaller video size and throttle that connection so that you're only served the smaller encode vs the full frame highest quality. Pretty simple thing to do really