I like to apologize, but I totally missed publishing the source code for the already published chapters of Programming with C++20 - Concepts, Coroutines, Ranges, and more.
The source code is now available at GitHub: github.com/andreasfertig/programming-with-cpp20. I hope this helps you and reduces the amount of code you have to type yourself from the book. I will update this repository with every new chapter I publish or any update I do for the existing chapters. However, if you catch me forgetting about it again, please tell me :-)
I plan to publish some updates to the book next week.