Edit backend?

I don’t know if I’m going to use the right term – or the right topic title, but what I’m wondering is which file(s) would a person have to edit if you wanted to change the html code that is used in the backend (ie: the code that wp is using for a particular element – like the
<li> tags that get inserted automatically into the menu’s? I’m often finding myself wanting to change the way something is displayed, but can’t (or don’t know how to) do it through the template and stylesheet.

But beyond that, curiosity being what it is, I want to know how/where/why/when this code comes from. Is there an easy answer?