An important thing about all controls in XAML is their "Content Property."

"Content property can only be set to an instance of another object." And it can be set only once but that’s not the case with layout controls. But Content Property of layout controls can be set more than once. In fact they don’t have a content property instead they have a "Children Property". Like Grid Control.

Children Property is a special data type (a collection data type) that can hold XAML controls called UI element collection.

In XAML as we add new instances of controls inside of the definition of our layout control, we are actually calling the add method of our layout controls. So here you can understand that how XAML is simplifying stuff for us.

Grid Control

Grid Control