Log into iCloud.com, find the video in your Photos library, and click the Download icon.
You might be missing the HEVC codec if the video was recorded in "High Efficiency" mode on an iPhone.
The .MOV format (QuickTime) is high quality but can be bulky. Converting it to .MP4 makes it easier to share on social media or play on older Windows machines. Open VLC and go to Media > Convert / Save . Click Add and select your IMG_6580.MOV file. Click Convert / Save .