You are getting into hard-mode async Rust now, so I can't give a comprehensive answer in a text box, but I can link some resources you might find useful:
- The last few chapters in the Tokio tutorial, starting from Async in depth
- Pin and suffering
- The os-phil post on Async/Await
The last post is pretty long, but it goes in a lot of detail on how async/await fits together.
Depending on what you need, it may be possible to combine tokio_util::udp with tokio_util::io::StreamReader and sidestep the poll stuff entirely.