On the sidewalks; hard to see in that light, and the picture might have been taken in autumn or winter, but I replied elsewhere in the thread with a picture showing how many of them there actually are (or just look up pictures with the keyword “eixample” and you’ll find there’s actually quite a bit of green between the blocks).

(And also parks, of course; there’s two of them in the picture right next to the basilica, but, again, in this yellow light you can’t see the green.)