Or more accurately, you now have to understand both the nginx.conf file format, and the right way to nudge the YAML so it produces what you want. Plus some intermediate wrapper script that sets things up on container start that does more funky things based on environment variables.

I don't get it, how do you configure nginx with a YAML file?

You don't directly. You add some extra lines of yaml to an k8s ingress resource, nginx detects this and updates itself. A single nginx container can service many ingresses for each of your apps. The idea is to distribute the config to the app manifests they are related to