File Packages ?

So I was doing some research because I wanted to make a file extension like .docx which is basically an archive with information for the app to read and write to but there's little to no documentation on how to do this or maybe I'm not searching it up right. All I've found was this https://www.pluralsight.com/guides/creating-file-packages-in-c and the code has some errors in it, can anyone help ?