Missing introduction of concept like "bases:" and "base" in Go-Getter url at https://repo1.dsop.io/platform-one/big-bang/customers/template

At https://repo1.dsop.io/platform-one/big-bang/customers/template, I'm unable to make sense of the YAML section starting with "bases:" I looked up docs of "Go-Getter", Kustomize, Flux 2, and still couldn't figure this out, and the "base" part in the URL -> https://repo1.dsop.io/platform-one/big-bang/umbrella.git/base/?ref=v0.0.*

Is "base" a real branch or is this substituted during the Kustomize transformation?