It’s new to me, I think it’s saying that your system is built up by you declaring what you want in a file, a single source that everything comes from.
It’s atomic because each action the system takes is carefully completed rather than bailing out and requiring you to fix something.
It’s immutable meaning you declare how you want things to be set up and then critical changes stem from those declarations and nothing else. You would obviously generate preferences, save data, etc. but the files that make the system / packages work are carefully locked.
It’s like the concept of flatpaks + structured system defining + modern common sense OS operations?
There are tracing programs that let you see when a program makes system calls to read and write files, control hardware, etc. It might be easiest to run it and see what it does in a VM sandbox. Process Monitor looks like a strace equivalent on windows.