iPhones record MOV files, by default in HEVC (“High Efficiency” in camera settings). They play fine on a Mac, but may not open on Windows, older Android phones or smart TVs, and some sites refuse them.
The converter checks what's inside first. If the video is already H.264, it's simply repackaged as MP4: done in seconds, with no quality loss. If it's HEVC, it's re-encoded to H.264, which every device understands.
HDR video (Dolby Vision or HLG from iPhone) is tone-mapped to standard SDR so it doesn't look washed out on regular screens.