How do you even go about doing this?
I'm talking about the tech tree representation you see when hitting F12 ingame, they buffed it in the latest patches so it looks better but I can't see a trigger/data method for changing anything, is it defined in some kind of hidden XML file?

Most of it is handled through the Unit data object in the data editor - I believe. But it might be possible to edit this manually through the uiLayout files. Not 100% sure though.

The only thing I could change from the data editor is what units are displayed under a production building, nothing else like the buildings' placement in the diagram or what buildings appear in the first place.
Where should I look to see if it's in a layout file?

http://www.galaxywiki.net/index.php?title=UI_Editing it's incomplete, but on the list there are TechGlossaryPanel and TechTreePanel. I assume it'll be TechTreePanel.

Hey thanks, I managed to extract the TechTreePanel layout following that guide but I'm not sure if I'm importing the edited one right, I'd like to use the UI - Custom Layout Files method so I tried that but nothing happens, are you supposed to import the files with a special path or something? Tried both UI/Layout/UI and just a random folder but no go.

Import the SC2Layout files with the correct filename to the following path "UI/Layout/UI".