5 · Saving it
A web page can't write an MP4 on its own, so record the screen: press F for fullscreen, start a screen recorder (OBS, QuickTime, Xbox Game Bar, or your phone's recorder), let it run two full loops, then trim. Pick the aspect you actually need before recording — 9:16 for Reels/Shorts/TikTok, 4:5 or 1:1 for feed posts, 16:9 for YouTube.