"Nested layouts" just sounds like nested React components.

React is kinda weird… you have to do subcomponents the "React way" and I think a lot of folks struggle with that; it can feel like fighting. But I don't think Next makes that any easier or harder.

I don't know what you mean by "nested routes." Routes are just URLs that map to files. You can create a directory with files, and, uh, subdirectories with files, if you want…?

In Next, every route blows away the DOM and starts over. You can’t have nested routes like you can in React Router, where say you have different layout shells for logged out / logged in views, and then master/detail views where you navigate down a hierarchical navigation.

Well you sort of can. Just have a [view].js file, where the view variable will be the component. So you can have {view === 'details' && <Details />}

I’m guessing they mean pieces of state based on urls, like which tab is active here or is a modal open or closed kind of thing.