...are one world with three garments. For in the material world we live in, we have the body and its clothes, and the root of this must exist above. Now the clothes are what covers the body after it is already complete in all aspects, for the covering is not a part of the body. There can be many garments, one on top of the other, and not one of them adds any new aspect to the body.
The same is true of Beriyah, Yetzirah and Asiyah in relation to Atzilut. For we already know that the entire government is brought to completion in Atzilut, and it is here that all the couplings (זיווגים, zivugim) take place, producing all the different aspects needed for all the creatures in the world. However, this is not revealed as such in the lower worlds except through a garb of garments, these being Beriyah, Yetzirah and Asiyah, which are nothing but veils concealing Atzilut.
As you have already heard, Atzilut, Beriyah, Yetzirah and Asiyah are all one system. They are not merely separate levels of a hierarchical scale but a system in which these three serve to conceal the light of Atzliut and garb it so as to bring it down to the lower levels.