How does BB and OS work?

Hello all!

I am new to beagle bone and was wondering if anybody had any sources or information about how the operating system Angstrom works with the hardware (LEDs, GPIO pins, etc.) of the Beagle Bone. For example, when one changes the value stored in a file say for led usr0, does a virtual node layer in memory activate an ISR which in turn gives the particular pin a HIGH signal?

I’d like explanations on that level if possible! I am having trouble online finding sources for this.