Adaptive bitrate streaming

The player receives a set of renditions of one video at different bitrates and resolutions, cut into short segments. As it downloads, it measures how quickly segments arrive and picks the next one to match, dropping to 480p when a connection wobbles and climbing back to 1080p once it recovers. HLS and MPEG-DASH are the two protocols doing this behind almost everything you watch. The consequence for a creator sits upstream: you send one clean high-bitrate feed and YouTube builds the ladder from it, so pushing a stream your upload speed cannot sustain degrades the picture for everyone at once.