I think (!?) this interesting learning experience started with Envoy Go Extensions.
IIUC, Cilium contributed this mechanism (Envoy Go Extensions) by which it’s possible to extend Envoy using Golang. The documentation references BPF. Cilium and BNF were both unfamiliar technologies to me and so began my learning. This part of the journey concludes with Weave Scope.
I learned that Cilium is a CNI implementation that may be used with Kubernetes. GKE defaults (but is not limited to) to Google’s own CNI implementation (link).