A layer, not a policy of its own
The umbrella does not replace your auto or home liability cover; it rides on top of it. You keep at least the primary limits the carrier requires — usually a few hundred thousand dollars on each — and the umbrella pays only what exceeds them, up to its own limit. Because it is exposed only after the primary policies are exhausted, it is cheap for the size of the promise: a million dollars of extra protection typically costs the price of a couple of dinners a month.
On this page the annual premium is the rate per million times the number of millions you choose, and the layer it adds is simply the coverage amount. This page prices the first million on the same flat line as the rest, which is how these policies are commonly quoted, even if the first million can price differently in practice.