Agriculture is the cultivation and breeding of animals, plants and fungi for food, fiber, biofuel, medicinal plants and other products
I thought it was just plants and land.  Animals too?
No. They are used to sustain and enhance life.
Is most of it done on farms, or are there indoor facilities that help life too?
Modern agronomy, plant breeding, agrochemicals such as pesticides and fertilizers, and technological developments have in many cases sharply increased yields from cultivation, 
Aren't they trying to grow hydro plants too, where no soil is needed?
Yes. Selective breeding and modern practices in animal husbandry have similarly increased the output of meat, but have raised concerns about animal welfare,