it doesn't have to vary. I'd say a project where different page types are built by different teams and where these pages are only connected via links would quality as a micro frontends architecture.
My books starts with exactly this example .
However, for many use-cases this very simple approach will not be sufficient. You might want to show functionality from different systems on the same view. To enable this we need a composition
technique. The most basic one is to use an iframe, but you can also use solutions like AJAX, Server-Side Includes or Web Components to provide a more performant and seamless user experience.
If your use-case requires client-side rendered page transitions
for an app-like user experience you might want to move from links (hard navigations) to an overarching client-side router (aka application shell) that enables soft navigations. Then you're building a Single Page App that consists of Single Page App. I'll address all of these topics in the main part of the book.
I've also set up a little site, The Tractor Store, where you can see the running example code from the book. Example #1 is "Links & Pages"