configmap.yaml defines a ConfigMap in the ingress-nginx namespace named nginx-configuration. This controls the global configuration of the ingress controller, and already exists in a standard installation. The key proxy-set-headers is set to cite the previously-created ingress-nginx/custom-headers ConfigMap.
The nginx ingress controller will read the ingress-nginx/nginx-configuration ConfigMap, find the proxy-set-headers key, read HTTP headers from the ingress-nginx/custom-headers ConfigMap, and include those HTTP headers in all requests flowing from nginx to the backends.
Test
Check the contents of the ConfigMaps are present in the nginx.conf file using: kubectl exec nginx-ingress-controller-873061567-4n3k2 -n ingress-nginx cat /etc/nginx/nginx.conf