Are all SwiftUI Views intermediaries for UIKit views?

Absolutely NOT.

First of all, there is no such thing as UIKit for other Apple platforms. So for example, you will see other counterparts like AppKit for macOS, which is enough to say NO to your question.

Second of all, Apple is hardly trying to build all SwiftUI components natively. But until all of them are available, they are using other platforms' existing components instead.

_{Maybe they will decide to not do it someday, who knows...}

This repository can help you see which is linked with which for now