Monogame still uses the Microsoft.Xna namespaces to avoid breaking everyone’s code. See this thread. Vector2 and Texture2D are classes in the Monogame framework that had equivalent function in the XNA framework, so their names will be the same as the classes in the XNA framework. So you might see calls like Microsoft.Xna.Framework.Graphics.Texture2D in Monogame code, but that’s just because Monogame uses an identical namespace. XNA doesn’t have to be installed anywhere.
Hopefully I didn’t misunderstand your question.