Different individuals see "Mission Control", as it's called, differently. Angels presumably see it as God's Throne Room, to others, it's Eden, and to Dean and Sam, it's the botanical garden where they went on a class trip. It has a gardener, named Joshua.
Sam and Dean are shot, and are sent to Heaven. Castiel later contacts Dean, telling him to find Joshua, as he talks to God, apparently. They went to search for the Garden. One sees what they desire in The Garden according to Joshua; the boys see it as Cleveland Botanical Garden, where they went on a school trip.
Zachariah was in charge of the garden and had higher authority than Joshua, as Joshua stated that Zachariah could fire him.