The default /user page is tabbed with one tab being for 'Create new account' and the other being for 'Log in'. I don't need to change this a ton. I just want to add some text above the tabs. However I can't seem to find the template that controls this page so I can make a duplicate. How would I go about accomplishing the above?
I would add a block with the text / content you need. Then under the "Visibility Settings" for that block, choose "Only on the listed pages".
Obviously you can then set "user" as the path you want that block to show on, and any other pages you need that to show up. Make sure you put that block in a region that shows ABOVE the "content" region (which is where the tabs would show up).
Remember to clear the cache! [Development --> Performance --> Clear all caches]
Since you're just wanting content ON that page above the tabs, there's no reason to try and manipulate the actual tabs. Blocks seems to be the answer here!