They are made by madreñeros. Years ago there was a whole village in every county where all the neighbours dedicated their lives to make these shoes. You could easily place the origin of a pair of wooden shoe just by looking at the design and the décor that is often carved but sometimes painted. The design is usually different for men and women.
In the pub in Oviedo where the language exchange sessions are held (much better place than a library) there was a pair of madreñas. We didn’t know what they are so our new Spanish friends explained it to us. I could even try them on although they weren’t designed for flip-flops so I put them on without. Or rather, my Prince Charming helped me with it!