I've noticed that seemingly everything is saved to the SD card in the 3DS. Does the system itself have any memory itself? Or does it use the SD in it as a sort of storage for everything? And if the system does have any memory, how much storage does it have, and is it possible to access it?
The 3DS does have internal storage.
The system includes 1 GB of internal flash memory manufactured by Toshiba, but it is mostly used by the operating system and its pre-installed applications.
[The Home Menu] is used to launch software stored on Nintendo DS and Nintendo 3DS Game Cards, applications installed on a SD card and DSiWare titles installed in the system's internal memory.
These DSiWare titles can be stored on the SD card, but they can't be run unless they're on the internal memory (the System Settings will allow you to move them). From the original 3DS operations manual (English, p. 72):
You cannot start DSiWare software from an SD Card. To play a DSiWare application, please copy it to System Memory first.