Are you approaching this from an "I want scrolling to work right in my emulator" sense?

There's an internal 16-bit pointer into VRAM which is modified by writes to $2000, $2005, and $2006. Loopy's documents show which bits in the even/odd writes to $2005/$2006 modify which bits of that pointer. It then discusses how the pointer is modified during non-VBLANK. All the Weird Scrolling Tricks wind up falling directly out of those patterns.