Two ways Arduino && 8051

I am trying the things which the many of people already did many times with Arduino.... It is very easy to do the same things others have done But Sometimes i got this thought "What the Hell iam really doing with this Little arduino Chip".
Really it doesn't making any sense by using all the Makeup commands in the Arduino. I wanted to know what iam really doing with this little chip.
So i tried to figure-out something in Data Sheet But The Datasheet for Atmel328 Controller is Freakshinly big. It is almost more than 600 pages.

So I changed my mind to go through arduino datasheet and gone for 8051 family.

I don't wanted to go forward using hands on examples first i wanted to learn something about 8051 architecture.
what it is?
how every thing is working?
just wanted to learn every thing understandable to me.

and Whatever i write here is not my own brain... Its a collection of many websites and blog's i gone through while i am digging the net about 8051.