Suggested project layout:
/ProjectName /scapix /source /ui /ios /android /macos /javafx /web /etc... /platform (optional) /ios /android /macos /windows /web /etc...
/scapix - installation of scapix library.
/source - your shared cross-platform C++ source code.
/ui - subfolders with different versions of UI code. These may or may not have a one to one correspondence to a particular platform. You may, for example, have a production version of UI for macOS written in Swift and a test version of UI written in Java with JavaFX library, which can run on many different platforms, like Windows, macOS and Linux (it will not look truly native, but for a test UI this is probably fine).
/platform (optional) - subfolders with platform dependent low level source code. Most of the time this isn’t necessary, but if you need to access some platform specific API from your shared C++ codebase, this is the place to do it.