Unlike the First and Second Temples, which were built by human beings, the Third Temple will be built by God, in Heaven. After bringing it into the world and standing it in its place, He will also build the walls of Jerusalem.

The deeds of the Holy One, blessed be He, are not like the deeds of human beings. When human beings built the earthly Temple, they first built the city walls, and in the end, they built the Temple. The city walls came first, in order to protect them, and afterward came the building of the Temple. The Holy One, blessed be He, is not like this. He builds the Temple first, and in the end, when he brings it down from heaven and puts it in its place, He will build the walls of Jerusalem, the city walls. (Zohar 2:108b)

The Temple is like the heart, from which all other organs receive their vitality; the Temple gives life to the entire world. This is why the Temple is in the center of the world, just as the heart is in the center of the body. (Maharal, Gevurot Hashem 71)