MOV is the format of iPhones and QuickTime. Some Windows programs, Android phones, TVs and upload forms do not accept it. MP4 with H.264 video plays almost everywhere, and the file usually becomes smaller too.
Recent iPhones record in HEVC (H.265). Your browser must be able to decode HEVC to read these files: Safari and Chrome or Edge on most recent computers can. If the video cannot be read, set the iPhone to Settings › Camera › Formats › Most Compatible for future videos.