How can I make my own mesh and add it to Addmenu > Mesh? As Blender is an open-source Program of coarse I can Edit its program files !

I want to make my own mesh (Car/Bus/) and I want it become a a regular mesh In add menu like a cube,sphere (Because I use it many times)

Why?

I do not want my mesh to be in file and every time i want to use i must imported.

How Can i Make it ? - Of coarse it will be made with Python - ( Are n't I right ? )

N.B :
Plese in answer Be ware that i am new to blender python. bpy.

Refrence

Consider that this is my mesh:


And I Want Its Shortcut in This Menu Here:
You'd have to create your own add-on for this. First create a Python function that creates the mesh, then create an operator to call that Python function, then add the operator to a menu. There are loads of online tutorials on how to do these things; the Blender API documentation is a good start, as it also includes information about best practices and common pitfalls.

You can check out the built-in Add Extra Objects add-on to look at similar functionality.