Shorts & Vertical Video
Collab
Also called: Shorts Collab, side-by-side remix
Collab sits behind the Remix button on a Short or a long-form video. Pick it and you film your own Short while the original plays, choosing a layout where both run side by side or the original runs first and your response follows. Reactions, answers, before-and-afters and dance videos all use it, and the original creator keeps their attribution. Anyone who does not want their work remixed can switch the option off in the video's settings.
YouTube also has a feature called Collaborations, which credits several channels on one upload and surfaces it to each of their audiences. Same word, different mechanism, and the two get confused constantly.