integration - What is $f(rcos(theta), rsin(theta))$ equal to $1$ in this double integral?

Use a double integral to find the area of the region.
The region inside the circle
$(x − 2)^2 + y^2 = 4$
and outside the circle

$x^2 + y^2 = 4$.

I understand how to get the limits of integrand for this region on $r$ and $\theta$, but when you set up the integral it is as follows.

The general form of a double integral in polar is:

$$\iint f(r\cos(\theta), r\sin(\theta)) r \, dr \, d\theta$$

However when evaluating this integral it becomes:

$$\iint r \, dr \, d\theta$$

Can someone please explain why

$$f(r\cos(\theta), r\sin(\theta)) =1$$

here?