YouTube by default prefers newer, bitrate saving codecs over old ones if it thinks your CPU can handle software decoding them. On my 2017 Dell XPS 1080p and low

YouTube by default prefers newer, bitrate saving codecs over old ones if it thinks your CPU can handle software decoding them. On my 2017 Dell XPS 1080p and lower resolutions on YouTube play in software decoded AV1, only 1440p and higher play in hardware decoded VP9, so playing 4K video on YouTube is less taxing for my CPU than playing a 1080p video....