How exactly does the Amiga RTG system work?

[gazsp]

I was just wondering, when you use a graphics card with a big box Amiga, how exactly does the system know how to use the card?

Does graphics.library just get patched so that all OS calls go to the cards library / drivers instead of using the native chipset?

I'd just like to know has I don't have much understanding of the Amiga's RTG system.

[gazsp]

User selects graphics card screen mode for desktop and/or applications.

I know that much :-) Is it just the files in Devs/Monitors that inform the system of stuff like where the screen memory is located, what format the data is in (i.e. planar, chunky) etc. ?

I just don't really understand how it all gets set up initially in software so that the system knows to stop using (reading and writing to / from) planar Chip RAM, and start using the memory / GPU on the graphics card.