[How most the 'modern' HUDs works ?]

Looking the big complex huds of product like the ones provided with mesh body or mesh heads I cannot figure out how they could work.
I mean, there's a lot of clickable areas, with very complex shape (not just a square or a circle) and very close each other (without gap between each other).
As noob scripter I cannot figure out how they can function, there's no invisible clickable prim, they don't (I think) compare mouse coords with a matrix of point to detect what you are clicking, as far I see they don't pickup color from pic or do anything else I can imagine..

Anyone know how they get that result?
..if it's a too complex explanation to write down could you provide me a link, or an info source I can read / study to find the answer?

Thank you (in advance) !

[looking for this hair]

At my eyes is:

https://marketplace.secondlife.com/p/BURLEYBabsIISampler-wear/7332553

the red nuance.

Same shop hase the relative demo to "try before you buy" : P