Container versus codec
These are two different things that share a file extension, which is most of the confusion. The container is the box — MP4, WebM, MOV — and it holds one or more streams inside. The codec is the encoding of those streams: H.264, VP9, AV1 for video, AAC or Opus for audio.
You can have H.264 video inside a WebM file. The extension tells you the container; the codec tells you how the picture is stored.
Why MP4 and H.264
H.264 inside an MP4 container is the combination with the fewest surprises. It plays in QuickTime, in VLC, in a browser, on iPhones, on Android, and on most smart TVs without anyone installing a codec pack.
YouTube publishes plenty of VP9 and AV1, usually inside WebM. Those are more efficient, which is why YouTube prefers them, but they are also where playback problems come from: a file that stalls in a browser and refuses to open anywhere else.
The YouTube Video Downloader lists the H.264 MP4 renditions for exactly that reason. Choosing the format that opens everywhere is worth more than the smaller file size.
Audio, and why it is M4A
Audio-only downloads come back as M4A. That is an MP4 container holding a single AAC audio stream, no video. It plays on the same devices as everything else here, and it is a fraction of the size of the video.
YouTube also publishes Opus audio, which sounds better at a lower bitrate. Like the modern video codecs, it is skipped in favour of compatibility.
What resolution means
Resolution is the pixel count of each frame, written as width by height — 1920x1080 is 1080p, 1280x720 is 720p, 854x480 is 480p. The number after the p is the vertical pixel count, and that is what the quality label refers to.
Higher resolution means more detail, and more detail means a bigger file for the same viewing experience. How much bigger is covered in 1080p vs 4K video quality.
- 1080p — full HD. The safe default for most purposes.
- 720p — noticeably softer on a large display, but small files.
- 480p and below — for slow connections or older hardware.
Formats deliberately not offered
WebM, VP9, AV1 and Opus all exist on YouTube and none of them appear in the format list. That is a choice, not an oversight: the point of downloading a file is usually to use it somewhere other than a browser, and these are the formats least likely to open there.