Usually wrapped in an MP4 container, though it is technically "video-only" and must be merged with an audio stream (like itag 140) to create a playable video file. How to use it in yt-dlp

AV1 ( av01 ), which is a modern, royalty-free codec designed for better compression and quality compared to older formats.

It is the most modern codec YouTube offers for 1080p, allowing for higher frame rates that older caps (like 30fps) don't support.