Guide
Compress MP4 on iPhone
The app outputs MP4 using HEVC or H.264 when the device supports it. Processing is on-device.
Import an MP4 (or another common video from Files or Photos), choose a preset, and compress. Output is MP4 using HEVC or H.264 depending on the device.
Photos (JPEG, HEIC, PNG) are handled in the Photos tab of the same app.