I hate to say this, but it sounds like you're way over your head.
Every computer, and I mean EVERY computer, does the same thing.
Somewhere inside, there is a processor. Some are made by Motorola,
some by Intel, some by AVR, etc. etc. There is no real difference
between an embedded system and a "computer" (like a PC), except in the
use to which it is put. After reset, the processor puts an address on
the bus. Some hardware, somewhere, recognizes that address and comes
up with some data for the processor to use. The value of that data, in
concert with the exact processor in use, determines what happens with
that data. This cycle of events happens over and over, causing the
processor/system/computer to do something useful, if you're lucky :-).
Sometimes the address recognition hardware is inside the processor.
This is common in "microcontrollers". Sometimes the recognition
hardware is connected to a peripheral. For instance, if you wanted to
send a character down an RS-232 line, the processor would just write a
byte to a specific address. From the processor's point of view, there
is nothing to distinguish that address from any other address. Once
that byte is written, a separate piece of hardware (either internal or
external) converts it to serial, adds start and stop bits, converts it
to RS-232 voltages, and sends it out at the correct speed.
There are a wealth of books out there that can explain all of this, in
much greater detail. I have stacks of them at work. Get the users's
manual for a simple processor, such as the 68000. Look for the
pictures that show a "typical" system, consisting of RAM, ROM, and
peripherals. Now get the user's manual for a microcontroller. Find the
picture of the internal architecture, and compare it to the other
picture. Notice the similarities and differences.
Keep asking questions.