I'm wondering whether there's a best practice to create a mirror part?  I trying to do this simple mirror exercise and I'm not sure whether I have a wrong expectation of mirroring or it's something that I did wrong.

What I'm hoping is the first half of the part with the post on the left-hand side and when I mirror it, I'd like to have the mirror post on the right-hand side so that both can fit into each other.  Is that a wrong expectation on what mirroring is or I just did it the wrong way?

Any advise?