Normally you get a lunch pause which can take anything between 30-120 minutes. Up to you how long the pause takes, later you have to compensate obviously.

I often have been the only guy at the office who took naps at lunch time.

That's not true. The correct name is 'siesta' as well.

Not true at all. The concept of "siesta" exists, but nobody does it at work. It's mostly something for retired people with a lot of free time and not much to do.

So what do all the retail staff do when the shops close between 12 and 3?

Only small shops do this, and it's more of a 1-to-4 thing. I guess they could... I don't know, have lunch, spend time with their family, do some chores?

Besides, you can't expect a small store with one or two employees (e.g. a bookshop) to be open from 9 AM to 8 PM without any breaks in between.

Spaniard here. I've never seen any spaniard nap at work in Spain. I've seen people nap in chinese companies though (in Spain and in China).