As you say, you deleted all previous versions of Windows but I thought I'd mention the Windows.old folder just incase.
When you do an Operating System update Windows creates a folder on the C drive called Windows.old incase you don't like the new 1 or it's riddled with bugs etc. This folder stores lots of files from the previous version you had so you can revert to the previous version if you want to.
The Windows.old folder is automatically deleted after 30 days.
If you're happy with the new Operating System and want to stay on the new version you can delete the Windows.old folder easily. It usually contains about 15-20 GB, maybe that's where a lot of your SSD storage has gone.
https://www.techadvisor.co.uk/how-to/windows/how-delete-windowsold-folder-3490793/For the other problem about watching the movie, and this is just a guess, maybe the movie was added to some sort of cache on your laptop seeing as you used an external drive. Clearing your laptops cache might regain the storage you lost.