You can use universal media players like VLC Media Player, which can handle most MP4 codecs even if the file extension or metadata is slightly corrupted.
Be cautious when opening files with unusual or randomized names from untrusted sources, as MP4 containers can occasionally be used to hide malicious scripts or "steganography" in unused metadata fields. 43400grgrgngnnvrg0hrt.mp4
If the video contains a scene you don't recognize, you can take a screenshot and use a reverse image search tool like Google Lens or Yandex Images to find its origin or other places it has been posted. You can use universal media players like VLC