1. "Whose" refers to "Industrialized agriculture".
2. There is no subject. It is a participle, and the whole phrase "housing as many people on as little land as possible" is a participial phrase that acts adjectivally, modifying the noun "building". (If it makes it clearer, it is equivalent to writing "... buildings which house as many people on as little land as possible".)
I have to say, I find this a bit tricky to decipher myself. There's an ambiguity about the scope of the "and", which doesn't help. It could be:
a) ... the efficiencies of complex urban [infrastructure and buildings housing ...]
b) ... the efficiencies of [complex urban infrastructure and buildings housing ...]
c) ... [the efficiencies of complex urban infrastructure] and [buildings housing ...]
Or indeed some other variants. I think I could persuade myself, if not anyone else, that (b) might make most sense.