To download video from YouTube in small format / file size suitable for e.g. copying into Dropbox and watching on e.g. an iPhone
yt-dlp \
-f 'bestvideo[height<=360]+bestaudio/best[height<=360]' \
--write-sub \
--write-auto-sub \
--sub-lang 'en.*' \
--embed-metadata \
--merge-output-format mkv \
$URL
where $URL
is the URL of the YouTube video