Rrecord your screen with your phone's normal recorder, drop the clip in, and it drops it inside an iPhone mockup on a nice background with zooms, captions and music.
Nothing is uploaded — all the rendering happens on the device, so it's private.
No install, no account — open the link, add it to your home screen, it behaves like an app.
It actually exports — a real MP4 (or GIF) you can save to Photos and share.