How to manually select video and audio format while using youtube-dl

Please read the youtube-dl documentation:

By default youtube-dl tries to download the best available quality, i.e. if you want the best quality you don't need to pass any special options, youtube-dl will guess it for you by default.

Note that if you have a specific requirement regarding video and audio codecs or formats used, you should set the format:

You can also use a file extension (currently 3gp, aac, flv, m4a, mp3, mp4, ogg, wav, webm are supported) to download the best quality format of a particular file extension served as a single file, e.g. -f webm will download the best quality format with the webm extension served as a single file

YouTube will offer VP9-encoded video and Opus-encoded audio when querying a WebM format; these are the itags starting with a "2". As these are way more efficient (i.e., offer better quality at the same size) than the H.264- and AAC-encoded video and audio streams starting with "1", you could choose -f webm for any download.

Also, there are more options for querying the format.

Add the option --merge-output-format mp4 to your command:

youtube-dl --merge-output-format mp4 -f "bestvideo+bestaudio[ext=m4a]/best" --embed-thumbnail --add-metadata [url]

From the manual:

--merge-output-format FORMAT If a merge is required (e.g. bestvideo+bestaudio), output to given container format. One of mkv, mp4, ogg, webm, flv. Ignored if no merge is required

To make this work with webm audio formats:

youtube-dl --merge-output-format mp4 --postprocessor-args "-strict experimental" -f "bestvideo+bestaudio/best" --embed-thumbnail --add-metadata [url]

• Note that as opus in an mp4 container format is "experimental", some players may have issues playing back such files