Why is the center of mass of an L shaped object outside the object

this is because the body isnt of any regular shape. So, the centre of the mass cannot be canculated ffrom just observation so we need to do calculations and from calculations it is outside the body.

When the Center of mass is outside the body its outside the body there is no question like why it is not in the inside

It just is. The centre of mass of a doughnut is in the hole.