He didn’t explain it well. The whole system lives on a ymal file and is easy to read. Documentation as code. If you have a working system then you’re set, it’ll never break. Adding software uses it’s own dependencies and will never break other software. It also has roll back features like snapshot/btrfs, during bootup you can go back to a previous version of your system. With the ymal file it makes it easy to clone the setup from others or for other systems of yours in the future, just have to generate a hardware file in most cases.