Xamarin Forms Box Sizing

I have started doing development on Xamarin Forms recently, and one of the things that I keep thinking about is the Box Model in Xamarin Forms.

So basically, does padding and border count as part of the width, or are they added on top?

For example, if I do <Frame WidthRequest="100" HeightRequest="100" Padding="20,0"/>, does this mean the total space the frame will occupy horizontally is 140', similarly, will the total space it occupy vertically is 140? Or will it be only 100 both horizontally and vertically, while the padding is taken from the WidthRequest/HeightRequest size?

This is know as Box Sizing in HTML/CSS, but not sure what it is called in Xamarin Forms?

I am suffering lately especially working with Frames, which they seem to be inconsistent to me.

Thanks for help.