This naming convention is frequently used by platforms that need to give files unique names instantly without user input:
The string of numbers is a in milliseconds, which represents the exact moment the file was created or saved. Timestamp: 1565839234123 Date: Thursday, August 15, 2019 Time: Approximately 03:20:34 AM (UTC) 1565839234123 mp4
VLC Media Player is highly recommended as it can play almost any .mp4 variant even if the file is slightly corrupted. This naming convention is frequently used by platforms
The subject is a typical auto-generated filename for a video file, likely created by a messaging app or a screen recording tool. Breakdown of the Filename 1565839234123 mp4