"on top of" is an idiom that means "as well as", "in addition to" e.g. On top of all the problems that I mentioned, my car has now broken down! "on the top of" is literal e.g. I am standing on the top of a hill. Where are you?

“On top of that” means “in addition”, “moreover” while “on the top of that” means “ above” ：)

We use both. It depends on the situation, but sometimes they mean the same thing.