How Do Flatten and Union Affect Gradients in Figma?

Hi, I have a question about how gradients work in Figma.

When I use a Fill with a Gradient, what is the difference between using Flatten and Union?

This is what I understand:

-Flatten: If I select multiple shapes and use Flatten, they become one single shape. Then, the gradient is smooth across the whole shape.

-Union: If I use Union, it looks like one shape, but inside, each part is still separate.

So the gradient is applied to each shape individually, and the direction can look different for each part. Is this correct? I want to know how Figma really handles this in the background. Thanks.