AV1 is a video codec that allows for the playback of 4K video at up to 60 frames per second (fps) in the presence of smaller file-sizes compared to other similar formats. MediaTek has announced that its 2020 flagship SoC, the Dimensity 1000, supports it at a hardware level. Therefore, devices with this processor can use the YouTube app at the highest resolutions.
https://www.notebookcheck.net/The-MediaTek-Dimensity-1000-is-the-first-SoC-to-have-a-hardware-based-AV1-video-decoder.463314.0.html