Adding a new tab / General add on questions

[Aadä]

Hi,
sorry to create a topic for this but I really could not find any documentation on how to do that.
I have basic knowledge on php/html/sql, noob knowledge on javascript/css, no knowledge on xml, none either on lua but successfully custom some add ons as needed.

I'd like to add a tab in the collection frame, through an add on I would do, for my own needs first, might release it if I find it useful and if I actually do it.

I thought I could find, somewhere, the actual UI code that defines the other tabs, the Blizzard ones, Mount and Wardrobe for instance, and that I could actually copy paste the exact same tabs, and edit it as needed.
1)is it possible ?

2)Where exactly can I find this code ? Also should this be stored in my add on in a .lua file, or should it be a .xml file ?

Lastly, if I need to use the same look as the other tabs, I should obviously use the same backgrounds for instance, and those backgrounds are located somewhere, I guess you should find the right name and call that name in the code.
3)Where are they ? I extracted BlizzardInterfaceArt and BlizzardInterfaceCode, but I'm not sure what I'm supposed to do with all that mess of folders and files...furthermore the base Blizzard UI have to be somewhere so I really don't get why some guides tell to extract those folders.

can anyone help me understanding how it should be done properly ?

thanks