Shorts & Vertical Video
Related video link
Also called: Watch full video link, link a Short to a long-form video
Adding one happens in YouTube Studio. Open the Short, pick a public or unlisted video from your own channel, and save. Viewers then see a tappable link below your channel handle while the Short plays, and advanced feature access on the channel is needed before the option appears.
This is the cleanest route from a Short to the longer video it came from, and it beats the old habit of writing "full video in the description" in a caption almost nobody reads. Set it while the Short is still moving, because traffic goes wherever it points at the time.