_defaults.yml

#@data/values
---
#! default base of all endpoints
endpoints:
  default:
    base: https://default-server

  serviceA:
    base: ~
    path: /service/v2/A

  serviceB:
    base: ~
    path: /service/B

  serviceC:
    base: ~
    path: /service/C
  

config.yml

#@ load("@ytt:data", "data"); d = data.values
#@ load("endpoints.star", "service_url")

---
endpoints:
  serviceA_URL: #@ service_url(d.endpoints, "serviceA")
  serviceB_URL: #@ service_url(d.endpoints, "serviceB")
  serviceC_URL: #@ service_url(d.endpoints, "serviceC")
  

endpoints.star

def service_url(dvs, name):
   base = dvs[name].base or dvs.default.base
   return base + dvs[name].path
end

z-data-values.yml

#@data/values
---
endpoints:
  default:
    base: https://staging-server

  serviceA:
    base: http://new-server
    path: /as/new/path