-vf format=yuv420p chooses YUV 4:2:0 pixel format.

-c:a aac choose encoder aac to output AAC audio.

-c:v libx264 choose encoder libx264 to output H.264 video.

This will convert the MOV to H.264 video and AAC audio: ffmpeg -i input.mov -c:v libx264 -c:a aac -vf format=yuv420p -movflags +faststart output.mp4

Audio formats compatible with MP4 include: AAC, MP3 (playback support depends on player), Opus (new format, so not universally supported).

Video formats compatible with MP4 include: H.264, H.265/HEVC, AV1 (new format, so not universally supported), MPEG-4 (old format, not supported in HTML5).

If you only care about local desktop usage, you can leave that out.

-movflags +faststart makes an output file that usable for HTML5 streaming, for example, putting all the necessary info to begin playback at the start of the file.

Stream copy ( -c copy) is like a 'copy and paste' so the quality is preserved and the process is fast.

You can stream copy if the MOV file contains video and audio that is compatible with MP4: ffmpeg -i input.mov -c copy -movflags +faststart output.mp4

Imovie convert mov to mp4