MOV to MP4.

iOS records in HEVC to halve the file size. Windows ships without an HEVC decoder unless someone bought the extension from the Microsoft Store, so the file opens as audio only, as a black frame, or not at all. Converting the video track to H.264 removes the dependency entirely.

Because of the codec inside it, not the .mov wrapper. iPhones record HEVC (also called H.265), and Windows only decodes HEVC if someone installed the paid extension from the Microsoft Store. Re-encoding that track to H.264 fixes it permanently, which is what this tool does by default.

HEVC is the newer codec and makes files about half the size at the same quality, which is why phones use it. H.264 is the older one and is decoded in hardware by essentially every device made since about 2010. If a file needs to open on someone else's machine, H.264 is the safe answer.

Only when a stream has to be re-encoded, and then only slightly at the default setting. Anything already H.264, AAC, or MP3 is copied across byte for byte, which is mathematically lossless. The plan line tells you which of the two is happening before you commit.

AC3 is re-encoded to stereo AAC, because AC3 in an MP4 plays on desktop players and almost nothing else. If you know your player handles it, the advanced options will keep the original audio instead. Only the first audio track is kept either way.

Yes. Browsers have no AVI parser at all, so those files go through the ffmpeg fallback, which reads them and always re-encodes the video (AVI-era codecs like DivX and MPEG-4 part 2 have no place in an MP4). It is slower than a copy, and the tool tells you that up front.

Files that the browser can read natively are limited only by your machine. Files that need the ffmpeg fallback are capped at 1.5 GB, because that decoder is a 32-bit WebAssembly program and cannot address more than that alongside its own buffers. For anything larger, trim it first.