AV1 (AOMedia Video 1) is the the next evolution of the defacto video streaming codec across the internet. It's planned as the successor to the HEVC (H. 265) format that is currently used for 4K HDR video on platforms such as Prime Video, Apple TV+, Disney Plus and Netflix.

Who uses AV1?

AV1 was launched by Amazon, Cisco, Google, Intel, Microsoft, Mozilla, and Netflix with a specific focus on delivering high-quality web video. As the benchmark above shows, VVC (h. 266) provides even better results than H. 265 or AV1, but VVC is also patent-encumbered and requires royalty payments.

Does YouTube use AV1?

The TL/DR is that YouTube uses H. 264 for the vast majority of videos that might get watched a few hundred times, or even lower. Starting in the 3-5,000 range, YouTube starts to use VP9, with AV1 reserved only for videos that will likely exceed more than five million views or so.