Swami Dayananda in this quote is explaining how the actual basis of everything is atma (consciousness), and that the mind exists in consciousness and not the other way around.
See if you can follow the logic (comments in brackets):
"There is a common notion that in every heart there is a divine spark. Ātmā [consciousness associated with a body-mind] is looked upon as something located in the buddhi [mind/intellect] of every being. According to this, the buddhi can be construed as the basis [the capsule], the ātmā as the based [the encapsulated], with a basis-based relationship between the buddhi and ātmā. To eliminate any such notion Bhagavān [Lord; Krishna as God personified in the Bhagavad Gita] says, ‘na ca aham teṣu avasthitaḥ.’ While all of them have their being in me, I am not in any of them. We have hundreds of such paradoxes. The meaning is simple – all of them are in me [consciousness; God without attributes], but because you may think that I am located in them, I say that I am not. They are all located in me, that is, I am the 'sat' [absolute truth] of everything, like even gold is the sat of all things golden. In every gold chain there is gold. Gold is not located in the chain; chain is gold. The gold does not exist in chain, in the sense it does not depend upon the chain, which is only the nāma-rūpa [name and form]. The very existence of chain is because of gold. If the chain is, the ‘ is-ness’ is nothing but the ‘ is-ness ’ of gold [actual reality]. Therefore, gold does not exist in chain, whereas the chain exists in gold. One is satya [real], the other, mithyā [apparently real]. Satya does not exist depending upon mithyā but mithyā cannot exist without satya."
—Quote from "The Bhagavad Gita Home Study Course," vol. 6 by Swami Dayananda