Use a modern media player like VLC Media Player or MPC-HC , as standard Windows/Mac players sometimes struggle with MKV files.
It is the default format for OBS because it is crash-resistant . If your computer loses power or the software crashes during recording, the file remains readable up to the second the crash occurred. (In contrast, MP4 files often become corrupted and unusable if not finished properly).
The filename is a standard timestamped file generated by recording software, most commonly OBS Studio (Open Broadcaster Software) .