MOV to MP4

MOV and MP4 are wrappers, not video formats. MP4 was built from Apple's QuickTime file layout, and both carry the same H.264 or HEVC video and AAC audio inside. Converting one to the other is mostly rewriting the wrapper: the tool reads each video and audio packet out of the MOV and writes it into an MP4 untouched. That is why it takes seconds, and why the result looks exactly like the original.

The exception is the codec, not the container. Since the iPhone 7, the Camera app records HEVC unless you switch it to Most Compatible. An MP4 with HEVC inside is still a valid MP4, but Windows will not play it without a paid extension and some editors and upload forms reject it. For those, the tool decodes the HEVC and encodes H.264 instead. That takes about as long as the clip itself on a recent laptop and costs a little quality, so it is a choice you make, not something done behind your back.

Either way the file stays on your machine. The browser reads it, converts it in memory with WebCodecs and hands back the MP4. No upload, no queue, no watermark, and nothing you have not published crosses the network.