This layered approach is based on bitplane groups, so with AGA's 8 bitplanes, you can split screen up in e.g. 3 layers, with e.g. 4+2+2 bitplanes, giving 16 colors for front playfield and 4+4 colors for second and third playfield. The copper can be used to alleviate the low color count in the playfields. The cloud layer is probably done by modulating the horizontal scroll registers, so it also doesn't cost much. Additionally sprites can be used to build on the background layers.

Result is that it doesn't cost much blitter/cpu performance to drive the game background, but most is still available to drive the enemies, an important point here. With a simple PC VGA-type chunky display (which lacks copper, bitplanes, sprites, and horizontal scrolling), this type of game would not have been possible without a significant amount of CPU power.