You don't have to 'install' event handlers, there are no event objects, and there's no event loop like:
while (uiEvent = uiGetNextEvent()) {
uiHandleEvent(uiEvent);
}
if (Button("Click Me!")) {
printf("Button Clicked!");
}
How all this is implemented inside the immediate mode UI framework isn't relevant. The 'immediate mode idea' is only about the user-facing code which describes the UI, not the internal implementation (those can vastly differ, and could even manage a 'retained mode' widget tree under the hood).
No, not at all. It's to take explicit control of the event loop. It's right there, you have a loop and the buttons still fire events, you just have to check for them on every iteration of the loop.