Services like iCloud or Dropbox often use this naming convention when you "Export" or download files to a desktop.
Since it is a QuickTime movie, it is compatible with most modern media players: 2021-10-21 16.45.43.mov
Can be opened using the Movies & TV app, VLC Media Player , or by installing the HEVC Video Extensions if the video was recorded in High Efficiency mode. Services like iCloud or Dropbox often use this