[QUESTION] Understading the [freeroam] resource.

[Moony]

I decided to dive into the complex system of the freeroam GUI MTA brings by default. After making it a standalone resource, I began changing a few things. I noticed a few particularities that I wanted to understand.
As the first particularity, I noticed that the bottons do not have a fixed position. They change its position and size depending on the position and size of the other bottons, as well as the length of the text inside. They follow the same order as the text: left to right first, then top to botton.
I wanted to add a new button to change the walking style, but I'm having trouble finding the specific part where the buttons are created and told to behave as mentioned above (so I can recreate the same thing myself)

Inside the resource, there is a 'data' folder that manages the lists of the elements the player can choose with several .xml files: weapons, vehicles, skins, etc. I could create a similar list, but I don't know how to tell the .Lua to call a list from a different file.

I'm not sure how this resource is creating all the elements of the GUI; I'm used to a different style (I use 'guieditor' to create GUIs). I still need to do some research. But in the mean time, what do I need to create a button that remotely calls of a .xml list with walking styles?