Editing “Comment” area HTML

hello! WP noob here. I’m in the process of altering the design of my WP site from the default to my own design. I need to alter the comment area (form etc) that shows up below an article. Where does this code reside? I can’t find it for the life of me. Is it an include or is it all built on the fly by PHP (please tell me that’s not so!).