now when i do the same thing, i double click (or i do command+O) but nothing happens.
See if QuickTime Player is still the default application for .MOV files. Do a Get Info (⌘I) in the Finder on a selected .MOV file, and look for the ‘Open with’ section to see if it says QuickTime Player there.

Also check the ‘More Info’ section of the Get Info window to learn about codecs used. It should say H.264, AAC or H.265, AAC. If it says different codecs, then that may be a cause of incompatibility, but QuickTime Player would tell you so, if that were the case.
Please let us know what is says there.

In my Apps i see QuickTime 10.5 but i don't seem to be able to activate it with anything that would previously open up via QuickTime.
Also try drag-and-dropping the file onto the application icon. That should even work even if a different application is default for .MOV files. Let us know the outcome of that.
Somehow, i was under the impression that QuickTime no longer worked on a Mac since Catalina, but i'll be happy to know that I'm wrong.
The confusion may come from the abandonment of the older 32-bit QuickTime 7.x with Catalina.
QuickTime 10.x works just fine.
maybe a link to the correct QuickTime i can download again from somewhere?
QuickTime 10 is part of the macOS system software. There is no separate download for it, only the full Catalina. I seriously doubt that the issue is with the QuickTime software itself. It is probably something else. But before discussing every possibility, please answer the above questions, which may help narrow it down a bit.