Immediate mode GUIs are an extension of the simple display update loop: (1) update display, (2) check input, (3) goto (1). Immediate mode GUIs add a context parameter to the update function. In the draw context, the update function draws to the screen. In the input context, the update function doesn't draw; instead it checks if mouse clicks or keystrokes should do something.

For example, part of the update function might look like this:

    if (Button("Do That Thing")) 
        doThatThing();

In the draw context, the call to Button will draw a button on the screen and return False. In the input context, the call to Button will check if a mouse click fell within the area it would have drawn. If so, it will return True and the callback will run.

An immediate mode GUI is basically the opposite of a typical Model-View-Controller GUI because it deliberately mixes the Model, View, and Controller together into one function. Just one function draws the GUI based on whatever state it's in and whatever the model is. The same function gets reused in a different context to update the GUI state or handle callbacks.

I'd argue an ImGUI does't mix anything. It's just a library. If you want MVC it's up to you to implement your MVC and use your ImGUI as a library. Retained mode GUIs don't magically make your code MVC and ImGUIs don't magically make your code not MVC. That's entirely up to you.

Also ImGUIs are not multiple pass. That's in implementation detail. Unity's ImGUI is multi-pass. One of the most popular ImGUIs (Dear ImGUI) is single pass

https://github.com/ocornut/imgui