Why does my iPhone 11 sometimes record in .MOV mode and not MP4 and how do I then convert these .MOV files to MP4 files?

The file specifications for modern MOV (and M4V) and MP4 have a huge overlap. For most files, no conversion is needed. Renaming .mov to .mp4 will just work for modern files, for use when the .mp4 suffix is expected.

The subtle differences do not apply to what your iPhone records.