A very long answer from Chat, but might help you.

The project hasn’t become a normal folder. It’s still a RapidWeaver project package, but Finder is displaying the package contents instead of treating it as a single document.

From your screenshot I can see:

DefaultPageAttributes
Pages
Resources
SharedPluginData
RWUIState.plist

Those are exactly the internal contents of a RapidWeaver project package.

The most likely cause

Finder is set to browse into packages (usually in Column View or after “Show Package Contents”), or the package bit has been lost.

First thing to try

Select the project.
Press ⌘I (Get Info).
Under Name & Extension, make sure the filename ends with:
.rw8
(or .rapidweaver depending on your version).

If it already has the correct extension

Open Terminal and run:

xattr -l \"/path/to/YourProject.rw8\"

and

ls -ld \"/path/to/YourProject.rw8\"

This will tell us whether macOS still recognises it as a package.

An easier test

Can you answer these two questions?

Which version of RapidWeaver are you using?
- RapidWeaver Classic 9?
- RapidWeaver 8?
- Elements?
If you double-click the .rw8 file, does RapidWeaver open the project normally, or does Finder just open it like a folder?

One other thing catches my eye in your screenshot: all the other .rw8 files have the purple RapidWeaver icon, but this one has a blue folder icon. That suggests this one project has lost its package flag, rather than Finder changing a setting globally. If that’s the case, it’s usually quite straightforward to restore without losing any data.

my Rapidweaver project file is now a folder, how do I make back into a project file