I skimmed some of this, but are you asking why you need to clone in the closure? Because "async closures" don't exist at the moment, the closest you can get is a closure that returns a future, this usually has the form:
where F: Fn() -> Fut, Fut: Future
i.e. you call some closure f that returns a future that you can then await on. when writing that out it will look like:
|| {
// closure
async move {
// returned future
}
}
`make_service_fn` likely takes something like this and puts it in a struct, then for every request it will call the closure to create the future to process the request. (edit: and indeed it does, it's definition literally takes your closure and uses it to implement the Service trait, which you are free to do also if you didn't want to write it this way
https://docs.rs/hyper/0.14.4/src/hyper/service/make.rs.html#...)
The reason you need to clone in the closure is that is what 'closes over' the scope and is able to capture the Arc reference you need to pass to your future. Whenever make_service_fn uses the closure you pass to it, it will call the closure, which can create your Arc references, then create a future with those references "moved" in.
It's a little deceptive as this means the exact same thing as above, just with the first set of curly braces not needed
|| async move {}
This is still a closure which returns a Future. Does all of that make sense? Perhaps they could use a more explicit example, but it also helps to carefully read the type signature.