The Kubernetes object definition file is in YAML format789. Kubernetes objects are represented in the Kubernetes API, and you can express them in .yaml format7. You can execute kubectl get deployment -o yaml to get the deployment definition in a yaml format8.

An OpenShift route is a way to provision external access to a service. A route exposes a service at a host name, such as that external clients can use to access the service.  A route can also specify security options, such as TLS termination, and load balancing options, such as round robin or least connection

Overlay networks are indeed virtual networks. They are logical constructs that stitch together disparate, dispersed network infrastructure, often referred to as underlay1. Underlay networks refer to the physical network infrastructure, while overlay networks implement network virtualization concepts2. References from Juniper site: Baeldung on Computer Science, PacketFabric, Cisco, HPE Aruba Networking