You’ve only scratched the surface of the AVR. There’s a world of details inside that little silicon chip.
If all this information has threatened to overwhelm you, take heart. You don’t need to memorize every fact and specification concerning these devices. It’s enough at this point to be aware of them in a general way. More important, you should have a good idea about where to start looking when you need more detailed information about a particular aspect of the system.
Download a local copy of the datasheet for your favorite AVRs, and take some time to browse through them, even if only to remember where the information can be located so you can look it up quickly when you need it.
Even if you don’t plan to write your own assembly-language ...