Two .mov files can have almost nothing in common.
MOV is Apple's QuickTime container. What's inside depends on what made it: a phone clip, a screen recording or an editing master differ in size by up to fifty times. Pick where your file came from to open the box.
In 2001 the MPEG group took Apple's QuickTime design as the basis for MP4. The two containers share the same structure of "atoms". That's why an iPhone MOV often converts to MP4 in seconds, just by rewriting the box around the video.
A MOV keeps its index, the moov atom, separate from the frames in mdat. Cameras write moov last, because they only know every frame’s size once recording stops. A player can’t show anything until it has moov. Drag the download and move the index to see the difference.
Inside moov, each track has a sample table: how long each frame lasts, which frames are keyframes, how frames are grouped into chunks and where each chunk starts. Seeking means walking those tables, then decoding forward from the last keyframe. Longer keyframe intervals make files smaller and seeking slower.
This page parses the atom tree itself. It decodes every track’s timescale, the edit lists that trim or delay playback, frame timing and keyframe spacing, and the display matrix that rotates phone video. It also reads timecode tracks and Apple’s metadata keys, location included. Then it can fix the rotation, strip the metadata or move the index to the front, without re-encoding a frame. Every rewrite is checked by following each new sample offset back to the same bytes. Everything happens on your device. Large files work too, because only the moov atom is read in full.
Identifiers from Apple’s QuickTime File Format specification, the four-character codes that name what’s inside each track, and the container’s history.
Each converter page explains exactly what changes.