I'll answer the polling question. The Tokio runtime (and async rust in general) works a bit differently than other async runtimes like node. With node, callbacks are provided and executed when an OS event is received. With Tokio, there are no callbacks. Instead, async logic is organized in terms of tasks (kind of like async green threads). When the task is blocked on external events, it goes into a waiting state. When external OS events are received, the task is scheduled by the runtime and eventually the runtime "polls" it. Because the poll happens in response to an OS event, most times, the poll results in the task making progress. Sometimes there are false positive polls.
This page goes into a bit more depth and shows an example of how one would implement a (very simple) runtime/executor: https://tokio.rs/tokio/tutorial/async
This page goes into a bit more depth and shows an example of how one would implement a (very simple) runtime/executor: https://tokio.rs/tokio/tutorial/async