I think every neighbourhood has its "own artist" who prepares them... Not sure about that, but I know it isn't an easy job to do, and they dedicate a couple of months to creating them...
If you are curious about them more, you can find some details here... https://en.wikipedia.org/wiki/Falla_monument

I didn't find the exact reason for burning, but I suppose it's to purify those bad/satiric ideas... 😃