if I understand correctly, smol was created by the async-std folks, and async-std now uses it as its runtime
Re:Tokio 1.0 – async runtime for Rust
by indepnd on Wednesday December 23, 2020 @05:07PM
(#25522819)
From what I've read, smol was created by its author and later on adopted by async-std. And, its author was in Tokio team, and later left and was in async-std team before he left and created smol. So smol was based on quite deep knowledge and experience in async.
The other popular runtimes are async-std [0] and smol [1].
[0]: https://github.com/async-rs/async-std
[1]: https://github.com/smol-rs/smol