If taken down by bulldozer, they take a lot of the roots with them. If cut down, there are stump grinders which are typically used to remove the bulk of them. From my experience, most don't take that job too seriously, often leaving some or much of the roots in the ground. In most cases, soil is brought back in to fill the area, usually humped slightly so it remains level after the inevitable settling in the pit, as the roots etc. are hard to compact properly.