SwiftUI isn't open source, so only apple employees know how it works at lower level, but I can link you to some of the newest features of Swift that SwiftUI uses a lot:
omitting return
property wrappers (for example @State)
Identifiable (we saw that it's in SwiftUI, and decided that it would be better if there was a type like that in standard library)
opaque types (it allows you to write some View)
function builders (Not officially part of Swift yet)
There's also a lot of Combine in SwiftUI, but that's another closed source apple framework