In the example below, note that some are audio only or video only.

youtube-dl -F https://www.youtube.com/watch?v=I8ik8pDTgJE
[youtube] I8ik8pDTgJE: Downloading webpage''
[info] Available formats for I8ik8pDTgJE:
format code  extension  resolution note
249          webm       audio only tiny   51k , opus @ 50k (48000Hz), 1.60MiB
250          webm       audio only tiny   68k , opus @ 70k (48000Hz), 2.11MiB
140          m4a        audio only tiny  128k , m4a_dash container, mp4a.40.2@128k (44100Hz), 4.22MiB
251          webm       audio only tiny  135k , opus @160k (48000Hz), 4.24MiB
160          mp4        256x144    144p   36k , avc1.4d400c, 30fps, video only, 507.10KiB
133          mp4        426x240    240p   64k , avc1.4d4015, 30fps, video only, 765.42KiB
278          webm       256x144    144p   75k , webm container, vp9, 30fps, video only, 965.44KiB
242          webm       426x240    240p  121k , vp9, 30fps, video only, 1.26MiB
134          mp4        640x360    360p  145k , avc1.4d401e, 30fps, video only, 1.40MiB
243          webm       640x360    360p  233k , vp9, 30fps, video only, 2.22MiB
135          mp4        854x480    480p  276k , avc1.4d401f, 30fps, video only, 2.34MiB
244          webm       854x480    480p  354k , vp9, 30fps, video only, 3.25MiB
136          mp4        1280x720   720p  484k , avc1.4d401f, 30fps, video only, 4.05MiB
247          webm       1280x720   720p  737k , vp9, 30fps, video only, 6.89MiB
137          mp4        1920x1080  1080p  784k , avc1.640028, 30fps, video only, 6.64MiB
298          mp4        1280x720   720p60  864k , avc1.4d4020, 60fps, video only, 6.56MiB
302          webm       1280x720   720p60 1068k , vp9, 60fps, video only, 9.96MiB
248          webm       1920x1080  1080p 1324k , vp9, 30fps, video only, 12.21MiB
299          mp4        1920x1080  1080p60 1514k , avc1.64002a, 60fps, video only, 11.66MiB
303          webm       1920x1080  1080p60 2000k , vp9, 60fps, video only, 17.48MiB
22           mp4        1280x720   720p  247k , avc1.64001F, 30fps, mp4a.40.2@192k (44100Hz)
18           mp4        640x360    360p  262k , avc1.42001E, 30fps, mp4a.40.2@ 96k (44100Hz), 8.71MiB 
(best)