Let's say we need to download videos from YouTube with the highest audio and video quality. Sometimes the combination is not available with most downloaders. But youtube-dl can download separate audio and video streams and mux them together. Here are some commands to do that.

# List all available streams for a video
youtube-dl -F "https://www.youtube.com/watch?v=abcd1234"
[youtube] abcd1234: Downloading webpage
[youtube] abcd1234: Downloading video info webpage
[youtube] abcd1234: Extracting video information
[youtube] abcd1234: Downloading js player en_US-vfl5-0t5t
[youtube] abcd1234: Downloading js player en_US-vfl5-0t5t
[info] Available formats for abcd1234:
format code  extension  resolution note
249          webm       audio only DASH audio   56k , opus @ 50k, 1.52MiB
250          webm       audio only DASH audio   84k , opus @ 70k, 2.10MiB
171          webm       audio only DASH audio  126k , vorbis@128k, 3.36MiB
140          m4a        audio only DASH audio  127k , m4a_dash container, mp4a.40.2@128k, 3.69MiB
251          webm       audio only DASH audio  161k , opus @160k, 4.03MiB
278          webm       256x144    144p  101k , webm container, vp9, 13fps, video only, 2.41MiB
160          mp4        256x144    144p  112k , avc1.4d400c, 25fps, video only, 3.19MiB
242          webm       426x240    240p  140k , vp9, 25fps, video only, 2.34MiB
243          webm       640x360    360p  234k , vp9, 25fps, video only, 3.98MiB
133          mp4        426x240    240p  248k , avc1.4d4015, 25fps, video only, 7.13MiB
134          mp4        640x360    360p  254k , avc1.4d401e, 25fps, video only, 6.49MiB
244          webm       854x480    480p  355k , vp9, 25fps, video only, 6.22MiB
135          mp4        854x480    480p  559k , avc1.4d401e, 25fps, video only, 13.74MiB
247          webm       1280x720   720p  601k , vp9, 25fps, video only, 11.70MiB
136          mp4        1280x720   720p 1171k , avc1.4d401f, 25fps, video only, 28.41MiB
248          webm       1920x1080  1080p 1220k , vp9, 25fps, video only, 23.32MiB
137          mp4        1920x1080  1080p 2258k , avc1.640028, 25fps, video only, 57.07MiB
17           3gp        176x144    small , mp4v.20.3, mp4a.40.2@ 24k
36           3gp        320x180    small , mp4v.20.3, mp4a.40.2
43           webm       640x360    medium , vp8.0, vorbis@128k
18           mp4        640x360    medium , avc1.42001E, mp4a.40.2@ 96k
22           mp4        1280x720   hd720 , avc1.64001F, mp4a.40.2@192k (best)

We can see that the highest quality video is 137 with 1080p 2258k and audio is 251 with 161k. But these are video only and audio only streams. Let's combine them.

youtube-dl -f 137+251 "https://www.youtube.com/watch?v=abcd1234"

It will pick a compatible format when merging. If mp4 doesn't work it gets converted into mkv mostly.

Misc

Automatically choose best video and audio: -f bestvideo+bestaudio
Download subtitle if the video has one already uploaded: --write-srt --sub-lang en