On its preview page for Apple TV 4K, Apple announced that the update to tvOS 14, which is expected to ship this fall, will support “YouTube videos in their full 4K glory.” By way of background, YouTube produces most 1080p videos using H.264 which plays everywhere but produces larger resolutions using either the VP9 or AV1 codecs. Apple never supported VP9 in any product or browser though it is a member of the Alliance for Open Media who developed the AV1 codec.
Apple doesn’t say whether the Apple TV 4K will support VP9, AV1, or both, or whether it will expand these playback capabilities to other products like Safari and the iOS. There’s tons of speculation on both these questions, but at this point, not a lot of definitive answers.
Apple TV 4K will at last play YouTube in 4K with tvOS 14 update – The Verge
tvOS 14 brings support for streaming YouTube in 4K on Apple TV – 9 to 5 Mac
Apple TV 4K will at last play YouTube in 4K with tvOS 14 update – Businesblog