I apologize for the confusing wording in the product. We recommend UI Builder and Workspaces for fulfiller use cases and Service Portal and employee center for requester use cases. For anything that requires catalog, pixel-perfect design, responsive design, or public pages we recommend using Service Portal/Employee Center. If by external users you mean users who aren't logged in, you won't be able to use your existing UI Builder portal anyway as none of our OOB data resources work publicly.