This is what I am trying to figure out too. Is Knative standalone, or is it running inside your K8 cluster, or do you build a K8 cluster and run this in it exclusively?
It's a whole lot of infrastructure to support a PaaS / FaaS workflow. From building and routing/serving/tracing to complex event driven stuff.
It's fully k8s native (hence the name, I reckon), and it uses CRDs. Which is basically k8s's DSL for describing anything. Basically a k8s-standard for k8s plugins with schema, validation, schema change management, kubectl support, etc.