Asio is a cross-platform C++ library, see https://think-async.com. It provides a consistent asynchronous model using a modern C++ approach.
Please refer to the original asio documentation at https://think-async.com/Asio/Documentation. Asio also comes with a number of examples which could be find under Documentation/Examples on that web site.
ESP platform port currently supports only network asynchronous socket operations; does not support serial port and ssl. Internal asio settings for ESP include
EXCEPTIONS are enabled in ASIO if enabled in menuconfig
TYPEID is enabled in ASIO if enabled in menuconfig