I was surprised that macOS (QuickTime/Preview, iMovie) can't read .mp4 files. Not sure if it was due to H.265 or the audio codec. I tried using ffmpeg to convert to .mov but that also failed to open, since I guess MOV is just another container format.
Is there an easier way?
IMHO the de-facto video player for macOS is [IINA](https://iina.io/).
Try something like: ffmpeg -i in.mp4 -c:v h264 -c:a aac out.mp4
To re-encode the content into H.264+AAC, rather than simply "muxing" the encoded bitstreams from the MP4 container into a new MOV container.
vlc
MP4 is container, not format, so if you have unsupported format packed into MP4 container it won’t be played. Example is trying to play AV1 video codec on devices with M2 chip or older. It won’t play. But it will play on devices with M3 chip and newer. Easiest solution is to use other player so that you can watch any MP4 file but with software decoding where hardware decoding is not available. Examples of such players are MPV or VLC.