Help please 
I’m trying to move my files from my old Windows to my new Mac. I’m using the .scrivx files, but when I try to open them on the mac it says ‘could not be opened’ and that it’s the 'main structure file for a project, but other required files for this project could not be found: the /Files/Data and /Settings folders are missing"
I also tried opening straight from my USB drive, the .scriv files and I get the same error.
help?
If your old Windows computer is still running: Open the project(s) on WIndows. Use File → Backup → Backup To… to create a new backup (this is the Mac menu path, I assume it’s in the same location on Windows). Make sure the option to use a Zip file is selected. Move the zipped backup file over to your Mac. Unzip the file and your entire project should be there. Lather, rinse, repeat.
Also, make sure that your Mac is not configured to store your projects in iCloud (sometimes referred to as Optimize Storage). This will break your projects and cause you a lot of grief.
Thank you!
For anyone that might be searching and having the same issue…
I did as scarlock suggested and then instead of using my USB drive, I simply emailed the zip file to myself and opened on my Mac. It worked. I’m not sure why it was different than the files I made before, but I’m not complaining. I have my writing!
What version of Scrivener on Windows, version 1 or 3?
SCRIVX doesn’t exist as a format on Mac.
Backup your project from within Scrivener on Windows as a zip. Move it over to Mac. Mac is supposed to open zip files without unzipping according to what I’ve read.
Click on the SCRIV folder/file in the zip on Mac and it will open your project.
In that case it also doesn’t exist on Windows. 
(The project contains a {projectname}.scrivx file on both platforms, macOS is just better at hiding everything inside of it.)
You are welcome.
The reason why it was different is because a Scrivener file is not really a file, but a directory full of sub-directories and files. The .scrivx file is the table of contents (so to speak) and the real data in the project is in the sub-directories and files. Under MacOS those details are hidden, making it look like a single file, but Windows does not have that capability, so it is easy to miss parts of your project when copying/moving it around. This is also why services like iCloud and others that want to help optimize your storage by keeping files in the cloud and serving them on demand break Scrivener - Scrivener needs all of those files and sub-directories available all the time to let you work with your project.
Aha, now I know, too, sort of.
Scrivener projects are folders on both systems (containing other files and sub-folders), but on macOS those same project folders are registered as “packages” and appear as single files. Makes it harder to dive into and accidentally just copy one file out of it. That’s basically the whole difference.
No, you should always extract the zip file. Double clicking on the .zip on a Mac will extract it.
Just to add to what @November_Sierra said, if you are really interested… Find the project file on your Mac, right-click on it and from the drop down menu select “Show Package Contents”.
You should then see all of the files and folders contained within the package, including a “project_name.scrivx” file. This will look very similar to what you see with a Windows machine, but Macs are better at hiding the complexity 
Do not under any circumstances alter any of these files but simply close the window and your project will be visible again as a single file.
I’m more intrepid—on Windows, wouldn’t know about Mac. I last owned one in 2016.
I recently changed my username, which messed up all my external Scrivener links, of which I have many.
I then ran a low-level text editor on my Scrivener projects, changing the username wherever found. Took all of 30 seconds and worked like a bomb.
Quote Kevitec57`
I’m more intrepid—on Windows, wouldn’t know about Mac
Being intrepid is good, until it all goes horribly wrong Fine if you know what you’re doing, but I’m hesitant to assume everyone has a great deal of technical knowledge.