It's explicit that the code is async, but how the asynchrony happens is completely implicit with async/await, and is managed by a runtime of some kind.

Kernel-style async code, where everything is explicit:

* You write a poller that opens up queues and reads structs representing work

* Your functions are not tagged as "async" but they do not block

* When those functions finish, you explicitly put that struct in another queue based on the result

Async-await code, where the runtime is implicit:

* All async functions are marked and you await them if they might block

* A runtime of some sort handles queueing and runnability

Green threads, where all asynchrony is implicit:

* Functions are functions and can block

* A runtime wraps everything that can block to switch to other local work before yielding back to the kernel