how did callback help with async in javascript

Because you cannot use asyncronous APIs without callbacks (or any other callback based abstraction like promise/future/coroutine)

Nothing polls state of suspended function directly, same for future/promise, simplest explanation that some other code calls callback/future/promise/coroutine resume. This code is some other thread, code, runtime, system event. Under the hood of all those events are poll loops, but it depends how deep it is, sometimes on library level, sometimes on runtime level, sometimes on system/os and on most low level CPU clock. Particular implementation just depends on nature of this async code

In case of JavaScript it's JS runtime dispatches network request, time intervals and expose this API as callbacks or promises

does that low level system machinery (sockets etc) expose APIs to register a callback with, which then puts an event on the event loop?

https://kotlinlang.slack.com/archives/C0922A726/p1565959076455900?thread_ts=1565946172.448600&cid=C0922A726 It depends, sometimes it provides callbacks, sometimes it's just polling for some resource, depends on implementation details and level of abstraction. For example you can use Java NIO, which uses system dependant APIs to provide real asyncronous IO, but you also can use standard io which is blocking, do not have any callbacks, so to use it you invoke, it blocks thread, but you can wrap it to some asyncronous API which under the hood starts thread and when operation is finished notifies callbacks

On Linux for example it most probably uses epoll, just Google it if you interesting about particular implementation details