Ask HN: How do you handle composite UIs?

In the situation where you have something like a shared header across multiple SPAs or domains, what strategies do you use?

In simpler terms, how do you do an iframe without using an iframe?

I'm roughly getting at the ideas described here[1], but wondering how people deal with it tactically as well as the interaction with front-end frameworks.

1: https://docs.microsoft.com/en-us/dotnet/standard/microservices-architecture/architect-microservice-container-applications/microservice-based-composite-ui-shape-layout