Laura Lieber, The Torah: A Women's Commentary page 491
According to the Zohar, "All the priestly garments were emblematic of supernal mystery" (2:231a). This mystical reading focuses on the layered nature of the vestments, which indicate a balancing of dualities: left and right, soul and body, male and female. The primoridal human whom G-d created (Genesis 1) -- this passage teaches -- was a singular creature both mae and female, but he two halves were joined at the back, facing away from each other. G-d separated this single entity into two, and "thus they [the first woman and first man] were brought face to face, [and] love was multiplied in the world." Unfortunately this physical division also led to emotional estrangement, jealousy, and strife. The unity of the ephod and breastplate (which were worn together) recalled the original unity of the first woman and the first man and by extention, of all humankind
(29) Aaron shall carry the names of the children of Israel on the breastpiece of decision over his heart, when he enters the sanctuary, for remembrance before the LORD at all times. (30) Inside the breastpiece of decision you shall place the Urim and Thummim, so that they are over Aaron’s heart when he comes before the LORD. Thus Aaron shall carry the instrument of decision for the Israelites over his heart before the LORD at all times.
Why is it called Urim VeTummim? Urim, which is based on the word or, light, is so called because it illuminates and explains its words. Tummim, which is based on the word tam, completed, is because it fulfills its words, which always come true.
How is it done? How does the Urim VeTummim provide an answer? The names of the twelve tribes were engraved upon the stones of the breastplate. These letters allowed for the answer to be received. Rabbi Yoḥanan says: The letters of the answer protrude, and the priest then combines those letters to form words in order to ascertain the message. Reish Lakish says: The letters rearrange themselves and join together to form words. The Gemara asks: How was it possible to receive an answer to every question? But the letter tzadi is not written within the names of the twelve tribes engraved on the breastplate’s stones. Rav Shmuel bar Yitzḥak said: The names Abraham, Isaac, and Jacob were also written there. The name Yitzḥak, Isaac, contains the letter tzadi. The Gemara asks again: But surely the letter tet was not written on the breastplate, since it is not found in the names of the Patriarchs nor in the names of the twelve tribes. Rav Aḥa bar Ya’akov said: Shivtei Yeshurun, the tribes of Jeshurun, was also written there. The word shivtei, tribes, contains the letter tet. In this way the entire alphabet was represented.
The Gemara raises an objection from a baraita: Any priest who does not speak with Divine Spirit and upon whom the Divine Presence does not rest is not consulted to inquire of the Urim VeTummim. As Zadok inquired of the Urim VeTummim and it was effective for him, and he received an answer; but Ebiathar inquired and it was not effective for him, and he did not receive an answer. As it is stated: “But Ebiathar went up until all the people had finished” (II Samuel 15:24), which is taken to mean that he was removed from the High Priesthood since the Divine Spirit had departed from him. The Gemara asks: If it is true that the letters of the breastplate protrude or even join together to form the answer, why does the High Priest need the Divine Spirit and Divine Presence to be with him? And if he has the Divine Spirit and Divine Presence with him, why does he need the Urim VeTummim? The Gemara answers: The Divine Spirit assisted the Urim VeTummim. In other words, the letters formed the answer only if the High Priest himself was worthy, but his divine inspiration was not great enough to provide an answer without them.