Is there documentation for integrating with a backend?
Is there any documentation for integrating the React app with a backend? I've downloaded the demo apps, but it looks like all of the components contain static data for the demo sites and the components don't accept props for passing the needed data.
You are right, all components are examples of UI.
It's not possible to write such documentation cause on the backend side users can use different frameworks/languages and it will take a huge amount time of for it (even after that, each integration is custom, and coverage of needs would be very poor).
For the Authorization module, if your API is ready, you can switch from fake to yours followeth the instruction: https://preview.keenthemes.com/metronic8/react/docs/docs/backend.
For the User module, we have the example of possible API (documented in our Swagger).