`async` is neither multi-threaded nor single-threaded by default. It all depends on the underlying runtime. Rust's Tokio runtime, for example, is multi-threade

`async` is neither multi-threaded nor single-threaded by default. It all depends on the underlying runtime. Rust's Tokio runtime, for example, is multi-threaded by default and makes progress on several tasks at the same time by using multiple threads.