How to download YouTube Shorts
A Short is a normal YouTube video presented differently. Its link works the same way as any other, with one thing worth knowing about the result.
The link
Shorts use an address of the form youtube.com/shorts/ followed by the video id, rather than the usual watch address. Both point at the same video and both are accepted.
If you are in the mobile app, the share button copies a youtu.be link, which also works.
What the file looks like
Shorts are shot vertically, usually 1080 by 1920. The resolution you pick refers to the shorter side as usual, so "1080p" on a Short means 1080 pixels across a vertical frame.
They are brief, so files are small — a 60-second Short at 1080p is typically well under 20 MB, and conversion takes a couple of seconds.
The music question
Many Shorts use music from YouTube’s licensed library. That licence covers playback on YouTube; it does not automatically extend to a copy you keep or re-upload elsewhere.
Saving one to watch is a different matter from republishing it. Re-uploading a Short with licensed audio to another platform is the case most likely to cause problems.