On a superficial level, a Yantra is a geometric representation of an aspect of divinity, i.e., a god or gods. On the deepest level of existence, a Yantra is a structure inherent in the field of Consciousness, the underlying basis of all existence. On that deepest level, Yantras are alive and dynamic. It might be compared to our skeletal system which has a precise structure, but still moves and is alive. In that sense, a Yantra and the corresponding god are one and the same. All Yantras are superimposed, with different personifications, different faces, of the same one God or gods.
Mantras are sounds inherent to the very nature of the structure of existence. Different Mantras correlate to different Yantras.
When those Mantras are recited in a precise manner, they enliven the quality, the value, of the corresponding Yantra. The technology of the implementation of those Mantras is called Tantra.
You might say that if Yantras can be compared to physics, i.e., nature, then Mantras might be compared to formulas, and Tantra might be compared to technologies that implement those formulas to enliven the value inherent in the nature of the Yantras. Take, for example, a Lakshmi Yantra. It could be said that that Yantra is Lakshmi. A Lakshmi Mantra would then enliven the dynamics and functionality of the nature of Lakshmi. And the implementation of that Mantra would provide a technology that would have a Lakshmi influence upon the world.