Suppose you are inside the proton of a hydrogen atom. this proton is pretty large for you and has a certain volume although you’re not sure how much because you cannot see its edges.
You know there is an electron near you that is attracted to the proton.
this means that the electon will try to go inside the proton, which it can since the proton is not contained by anything and the electron is not kept out by anything. However since there is no friction in an atom because there is nothing to cause it, the electron will fly through and come back and so forth.
if you then take the average position of the electron, it will be inside the proton because that is the point it keeps coming back to.