[boomlinde]

I would have guessed that they use quadtrees for this, splitting each non-leaf node into quadrants of more detailed maps as you zoom in.

[incanus77]

It's a quadtree of sorts, but is typically done via map projection (web mercator) math so that each tile is replaced by four tiles at the next highest (more zoomed-in) zoom level.

Number of tiles to cover the world at zoom z = 4^z