Screen recordings are wide, so turning them vertical means either fitting the whole screen on a background or cropping to follow the action. Zoomr does both (Fit and Follow).
Glossary
What is aspect ratio (16:9, 9:16, 1:1)?
Aspect ratio is the shape of a video, written as width:height. 16:9 is landscape (YouTube, websites), 9:16 is vertical (Reels, Shorts, TikTok), 1:1 is square (feeds).