Those 8-bit controllers with (exclusively) on-chip SRAM and Flash are indeed quite simple - they can be summed up as "Jump to 0x0000", but really are surprisingly complex once you get into bootloaders and interrupt service routines and so on.

The proprietary blobs and GPU on the Raspberry Pi make basically impossible to have a full understanding of what's happening. Instead, I'd recommend learning with the TI Beaglebone Black, which has an ARM Cortex-A8 with an MMU and lots of open documentation.