tl;dw - The background is defined with 20 bits, which are repeated or mirrored to make the background (eg 00000000111111110000 would either make the background 0000000011111111000000000000111111110000 or 0000000011111111000000001111111100000000). If you write to WSYNC ($D40A), the processor pauses until the start of the next scanline. Then you have to count machine cycles to make sure you modify the background at the right time to make the two halves different. Repeat on each scanline to fully define the background. There is no "framebuffer" per se