Cross platform compatibility

I installed the latest Windows version today. Information in this web site says it’s completely compatible between Mac and Windows without any conversion. In short, I will be able to work on the same project on my Mac Scrivener version 3 and my Windows Scrivener version 3. My work is stored in an iCloud folder regardless of the platform (or even the version in the past) so I expected to be able to load the project I created on the newly installed Windows version earlier today on my Mac tonight. However, while I can see lots of folders in the project folder I don’t see a project file to load. Why is this happening?