It's Galicia. The population is extremely disperse within the mountains and hills. It's a radically different ecological environment compared to the rest of the country, translated into a vastly different human environment. Whereas the rest of the country has big cities or towns separated several kilometers, Galicia is full of small villages and homesteads next to each other. There are 50,000 population centers (cities, towns, villages) in all Spain (except Galicia), and about the same amount just inside Galicia. The population though is not that big or concentrated, but very dispersed. Of about 47M people in Spain, only 3M live in Galicia. There are no "big" cities. The two major ones are about 300,000 and that's almost too much already.

It's quite similar to population density maps for Spain - you can see similar patterns here too https://twitter.com/simongerman600/status/1610307502553370627?t=1gcobn_UITwz6SNCZUtHAQ&s=19

All that big middle part with Madrid in the middle? No one lives there basically. Rural life has almost completely died, we even have a name for it ("the empty Spain"). Meanwhile in the Northwest (Galicia and to a similar extent Asturias) there are ten million of tiny villages where people still live and need transportation. I'm surprised by Cantabria ans Euskadi because o would have expected them to be similar.