QuickTime Player will open by showing an Open dialog window, to select a file to open. Press Cancel if you did not want to open an existing file at that time. The Open dialog window will show the last used location, but may be initially set to a special QuickTime folder on iCloud Drive. Use the sidebar to navigate to local directories.
Double-clicking an associated file would try to open it in the default application for that suffix: E.g. filenames ending in .mov, .mp4. It would then show the clip window, or show an error message why it couldn’t open it.
If you see QuickTime Player on the left side of the menu bar at the top of the screen, then the app launched as expected.
If you switch to other apps or to the Finder when QuickTime Player has no windows open, then the app semi-quits itself (no longer shown as active app in the App Switcher or in the Dock).