The portal is the door to the internal developer platform, not the platform itself. The catalogue answers what exists and who owns it, the documentation how it works, and the templates — golden paths — how to do it. The value is in what the door leads to: a portal with no self-service behind it is a catalogue with a nicer theme. Backstage, open-sourced by Spotify in 2020 and a CNCF project today, is the framework most start from.
Why the repo should own what the portal shows, and why the template is the portal's first real feature, is the subject of Backstage är inte plattformen — det är dörren till den (in Swedish). The Developer experience service page describes how we build portals, golden paths and self-service.