MP4 files: what is inside one, and what to convert it to

Because the container only describes tracks, two files can both be valid MP4 and share nothing else. One might carry H.264 video with AAC audio, the next H.265, the next AV1, and a player that handles the first can fail on the third while reporting the same `.mp4` extension and the same `video/mp4` MIME type. This is why "does MP4 work here" has no general answer, and why converting between MP4 and MOV or MKV is usually instant while converting to WebM is not: the first pair is a repackaging job, the second requires re-encoding every frame.

The `moov` index also explains `-movflags +faststart`, which this site applies to every MP4 it writes. Encoders naturally put `moov` at the end of the file, since track durations are only known once encoding finishes. A player streaming over HTTP then cannot begin until the last byte arrives. Faststart rewrites the file with the index in front, costing one extra pass at export and buying playback that starts immediately.

Naming is a convention rather than a rule. `.m4a` is an MP4 file holding only audio, `.m4v` is Apple's video variant, and `.mp4` is the general case — all three are the same container, and renaming one does not convert it. Renaming is also not a shortcut: extracting audio to M4A re-encodes it here rather than lifting the existing AAC track across, so it is a second lossy generation exactly like MP3 is, just in the same codec family.