This part of the handbook is Work-In-Progress.
libasync is a support library for supporting C++20 coroutines and C++ senders/receivers. It also provides many asynchronous primitives (e.g., asynchronous events, queues and mutexes). Managarm, being an asynchronous operating system, makes extensive use of the functionality provided by libasync.
The code for libasync can be found at https://github.com/managarm/libasync.