D:\PC\Downloads>youtube-dl.exe -F https://www.youtubecom/watch?v=YEI17W5eSMk
[youtube] YEI17W5eSMk: Downloading webpage
[youtube] YEI17W5eSMk: Downloading video info webpage
[youtube] YEI17W5eSMk: Downloading js player vflUpPEZ9
[info] Available formats for YEI17W5eSMk:
format code extension resolution note
249 webm audio only DASH audio 61k , opus @ 50k, 1.66MiB
250 webm audio only DASH audio 80k , opus @ 70k, 2.23MiB
140 m4a audio only DASH audio 127k , m4a_dash container, mp4a.40.2@128k, 3.76MiB
171 webm audio only DASH audio 140k , vorbis@128k, 3.74MiB
251 webm audio only DASH audio 155k , opus @160k, 4.36MiB
160 mp4 256x144 144p 12k , avc1.4d400c, 30fps, video only, 271.79KiB
133 mp4 426x240 240p 17k , avc1.4d4015, 30fps, video only, 352.97KiB
278 webm 256x144 144p 28k , webm container, vp9, 30fps, video only, 582.99KiB
134 mp4 640x360 360p 33k , avc1.4d401e, 30fps, video only, 587.74KiB
242 webm 426x240 240p 38k , vp9, 30fps, video only, 927.17KiB
135 mp4 854x480 480p 58k , avc1.4d401f, 30fps, video only, 967.80KiB
243 webm 640x360 360p 71k , vp9, 30fps, video only, 1.69MiB
136 mp4 1280x720 720p 99k , avc1.4d401f, 30fps, video only, 1.54MiB
244 webm 854x480 480p 115k , vp9, 30fps, video only, 1.68MiB
137 mp4 1920x1080 1080p 159k , avc1.640028, 30fps, video only, 2.48MiB
247 webm 1280x720 720p 202k , vp9, 30fps, video only, 4.77MiB
248 webm 1920x1080 1080p 309k , vp9, 30fps, video only, 7.43MiB
17 3gp 176x144 small , mp4v.20.3, mp4a.40.2@ 24k, 993.64KiB
36 3gp 320x180 small , mp4v.20.3, mp4a.40.2, 2.75MiB
18 mp4 640x360 medium , avc1.42001E, mp4a.40.2@ 96k, 4.26MiB (best)
youtube-dl.exe -f 248 https://www.youtubecom/watch?v=YEI17W5eSMk
[youtube] YEI17W5eSMk: Downloading webpage
[youtube] YEI17W5eSMk: Downloading video info webpage
[download] Destination: Public Enemies Otis Taylor - Ten Million Slaves [HD]-YEI17W5eSMk.webm
[download] 100% of 7.43MiB in 00:01
General
Complete name : D:\PC\Downloads\Public Enemies Otis Taylor - Ten Million Slaves [HD]-YEI17W5eSMk.webm
Format : WebM
Format version : Version 4
File size : 7.43 MiB
Duration : 4 min 7 s
Overall bit rate : 251 kb/s
Writing application : google/video-file
Writing library : google/video-file
Video
ID : 1
Format : VP9
Codec ID : V_VP9
Duration : 4 min 7 s
Bit rate : 234 kb/s
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Constant
Frame rate : 29.970 (30000/1001) FPS
Color space : YUV
Bits/(Pixel*Frame) : 0.004
Stream size : 6.93 MiB (93%)
Language : English
Default : Yes
Forced : No
Color range : Limited
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709
D:\PC\Downloads>youtube-dl.exe -F https://www.youtubecom/watch?v=tXV7dfvSefo
[youtube] tXV7dfvSefo: Downloading webpage
[youtube] tXV7dfvSefo: Downloading video info webpage
[youtube] tXV7dfvSefo: Downloading js player vfl7GcuOz
[info] Available formats for tXV7dfvSefo:
format code extension resolution note
249 webm audio only DASH audio 60k , opus @ 50k, 1.91MiB
250 webm audio only DASH audio 78k , opus @ 70k, 2.50MiB
140 m4a audio only DASH audio 128k , m4a_dash container, mp4a.40.2@128k, 4.48MiB
171 webm audio only DASH audio 141k , vorbis@128k, 4.58MiB
251 webm audio only DASH audio 151k , opus @160k, 4.81MiB
278 webm 256x144 144p 97k , webm container, vp9, 24fps, video only, 3.03MiB
160 mp4 256x144 144p 110k , avc1.4d400c, 24fps, video only, 1.88MiB
242 webm 426x240 240p 217k , vp9, 24fps, video only, 4.90MiB
133 mp4 426x240 240p 360k , avc1.4d4015, 24fps, video only, 5.03MiB
243 webm 640x360 360p 408k , vp9, 24fps, video only, 9.95MiB
244 webm 854x480 480p 753k , vp9, 24fps, video only, 16.25MiB
134 mp4 640x360 360p 758k , avc1.4d401e, 24fps, video only, 10.17MiB
135 mp4 854x480 480p 1265k , avc1.4d401e, 24fps, video only, 15.85MiB
247 webm 1280x720 720p 1417k , vp9, 24fps, video only, 24.10MiB
136 mp4 1280x720 720p 1532k , avc1.4d401f, 24fps, video only, 22.78MiB
17 3gp 176x144 small , mp4v.20.3, mp4a.40.2@ 24k, 2.77MiB
36 3gp 320x180 small , mp4v.20.3, mp4a.40.2, 7.75MiB
18 mp4 640x360 medium , avc1.42001E, mp4a.40.2@ 96k, 15.14MiB (best)
D:\PC\Downloads>youtube-dl.exe -F https://www.youtubecom/watch?v=SdzLl-XpJt0
[youtube] SdzLl-XpJt0: Downloading webpage
[youtube] SdzLl-XpJt0: Downloading video info webpage
[info] Available formats for SdzLl-XpJt0:
format code extension resolution note
249 webm audio only DASH audio 57k , opus @ 50k, 1.28MiB
250 webm audio only DASH audio 75k , opus @ 70k, 1.68MiB
140 m4a audio only DASH audio 135k , m4a_dash container, mp4a.40.2@128k, 3.18MiB
171 webm audio only DASH audio 140k , vorbis@128k, 3.23MiB
251 webm audio only DASH audio 149k , opus @160k, 3.31MiB
278 webm 256x144 144p 97k , webm container, vp9, 24fps, video only, 2.29MiB
394 mp4 256x144 144p 106k , av01.0.05M.08, 24fps, video only, 2.29MiB
160 mp4 256x144 144p 111k , avc1.4d400c, 24fps, video only, 2.37MiB
395 mp4 426x240 240p 218k , av01.0.05M.08, 24fps, video only, 4.56MiB
242 webm 426x240 240p 223k , vp9, 24fps, video only, 4.78MiB
133 mp4 426x240 240p 389k , avc1.4d4015, 24fps, video only, 6.38MiB
396 mp4 640x360 360p 397k , av01.0.05M.08, 24fps, video only, 8.16MiB
243 webm 640x360 360p 412k , vp9, 24fps, video only, 8.98MiB
397 mp4 854x480 480p 718k , av01.0.05M.08, 24fps, video only, 14.52MiB
244 webm 854x480 480p 750k , vp9, 24fps, video only, 16.23MiB
134 mp4 640x360 360p 827k , avc1.4d401e, 24fps, video only, 13.37MiB
135 mp4 854x480 480p 1245k , avc1.4d401e, 24fps, video only, 21.56MiB
247 webm 1280x720 720p 1498k , vp9, 24fps, video only, 29.76MiB
136 mp4 1280x720 720p 2336k , avc1.4d401f, 24fps, video only, 33.40MiB
248 webm 1920x1080 1080p 2689k , vp9, 24fps, video only, 48.35MiB
137 mp4 1920x1080 1080p 4343k , avc1.640028, 24fps, video only, 50.47MiB
17 3gp 176x144 small , mp4v.20.3, mp4a.40.2@ 24k, 1.97MiB
36 3gp 320x180 small , mp4v.20.3, mp4a.40.2, 5.51MiB
18 mp4 640x360 medium , avc1.42001E, mp4a.40.2@ 96k, 16.69MiB (best)
Using Ant down loader, I was able to download the top video at 720p and the bottom video at 1080p.
It could be something Google is trying to do to prevent people from downloading their videos (at high resolution at least). They probably noticed most people use websites for this. If they separate video from audio, the website would have to download bot files to their server, put them together and then send you the file. This would mean the server needs to use their own processors and memory to let you download in higher qualities. Also, it would be them who would be sending you the file, not YouTube, and thus you'd be wasting their bandwidth, not just Google's. It would be more expensive for websites that way. As a result, they would stop offering to download at high resolutions. This is why local downloaders don't have an issue with this. It's using your own processing power and memory, and still using Google's servers to download. (All just a theory though.)
About the file format, I already uninstalled everything, but I'm 99% sure it was MP4.
To download, simply go to the Firefox add-on "store" and search for it. Once you installed, you'll be prompted to install the Windows program and, if you say yes, you'll be taken to the download site. Install, reboot your PC (or just sign out) and start downloading.
However, since this thread is borderline illegal, read YouTube's TOS before moving forward.