Find documentation on, or analyze to figure out, the structure of Xcode project files

Does anyone know where there’s some documentation on the format of the Xcode project files?

If there isn’t, or if it isn’t adequate, would someone like to analyze the files to try to map out the structure?

Please add what you find or figure out to the wiki: https://gitlab.com/grantneufeld/XcodeProjectSecurity/-/wikis/Xcode%20project%20file%20structure