Downloaded videos not compatible with QuickTime

Howdy! I'm currently on a 2020 MacBook Pro with macOS Big Sur 11.2 and I've been trying to download videos from Google Drive, Vimeo, and websites that provide free stock footage for B-roll but when I try to open the files from Finder that file has the QuickTime Icon (no thumbnail image of the actual video) and when I double click on it to play it I get the error message that says "The document "(insert name)" could not be opened. The file isn't compatible with QuickTime Player."

I used to be able to take the same steps when I had macOS Catalina and the files would open right up with no problem. I've tried deleting the files, re-downloading them, deleting file again, re-downloading until the file shows the thumbnail of the video. When I see the file with the thumbnail of the actual video I double click it and it opens QuickTime and plays the video perfectly. But after a couple of seconds or minutes, the same file that once opened, now has the QuickTime logo as the thumbnail and when I double click it the error message pops up again even though the file previously opened without a problem.

I downloaded files that are .MOV, .MP4, .MP3 and I've had the same issue. I called Apple Support and they told me that I need to download a file converter and use that from now on. I downloaded a file converter and when I import the file that gave me the error of it not being compatible with QuickTime it tells me "The operation couldn't be completed. (Read error 1.)"

If someone has had this issue or has this issue currently I would love to see what solutions you have found to resolve this issue. Thank you!