I am a bit confused at the moment about select(2), which states in the summary:
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing
as far as I am aware many libraries and programs such as libuv and nodejs use select/epoll/kqueue/iocp for their event loop, which is used for their corresponding async/await feature (and async I/O?).
So, what exactly does synchronous multiplexing mean? Can I achieve async I/O using select? What exactly is the difference between synchronous multiplexing and asynchronous multiplexing?
You've got a parse error there. It is not synchronous multiplexing but multiplexing of synchronous I/O: select is used to multiplex synchronous I/O calls. read and write and such are called synchronous I/O because they will either block until the transfer is complete, or not do the transfer (non-blocking non-ready sockets for example).
This can be contrasted with truly asynchronous calls where the system call just initiates the transfer and it is completed in the background and a notification is given after the completion.
The nodejs and libuv are different beasts. Even though the I/O in C is possibly multiplexed and synchronous, it will appear as asynchronous to them - there is no blocking synchronous read calls because it all happens transparently on the C/library side.