Place in the breastplate the Urim and Tumim, and they will be over Aharon’s heart when he comes before Hashem. Aharon will carry the judgment of the B’nei Yisrael over his heart, before Hashem always.

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.

Because it illuminates its words - it explains its words:

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.

Protruding - The letters would protrude, for example the Ayin from Shimon, Lamed from Levi, Hei from Yehudah, to spell עלה (go up. And each letter would not move from its place, but only protrude in its place, and [the Kohen Gadol] would decipher them.

Reish Lakish says: they would combine. According to the opinion that they would combine, meaning that the letters would place themselves in the correct order, there is no problem. But according to the opinion that they would only protrude, there is a bit of an issue - since they protrude but are not in order, how could one understand how to combine the letters to receive an answer? It is possible to say that the letters protruded in series, and in the order that they protruded, that's how they should be combined.

The Urim and the Tumim were the twelve stones.

את האורים ואת התמים THE URIM AND THE THUMMIM — This was an inscription of the Proper Name of God which was placed between the folds (i. e. the two pieces forming the front and back) of the breast-plate through which it (the breast-plate) made its statements clear (lit., illuminated its words; מאיר from אור, light, this being an allusion to the אורים) and its promises true (מתמם from the root תמם, an allusion to תמים) (Yoma 73b).

... There is an additional question: we cannot identify the Urim with the Thummim, for Scripture states, with Urim and Thummim (Neh. 7:65). Furthermore, the words with Urim and Thummim are in the plural. They thus consisted of many objects. Rabbi Solomon says that the Urim and Thummim consisted of an inscription of God’s explicit name. However, he would not have said this had he seen the responsa of Rabbi Hai Gaon. There is a very deep secret behind the ephod and the breastplate. I will allude only to a part of this secret. Perhaps he who knows the mind of the Most High will understand it. ...The breastplate was folded, for the Urim and Thummim were kept there until such time as they were needed. They were then placed outside of the breastplate... The Urim were of gold and silver. This was not the case with the Thummim. The [importance of the] number [twelve] has been established by irrefutable proof from the upper and lower worlds. The intelligent will understand what I say. ...Now if I wanted to even begin to reveal this secret it would take up more space than the entire commentary which I have composed on this book. For only an individual who has studied works on geometry and the secret of the way the sky works can understand the secret of the ephod.

AND THOU SHALT PUT IN THE BREASTPLATE OF JUDGMENT THE URIM AND THE THUMMIM. Rabbi Abraham ibn Ezra thought to display wisdom in the matter of the Urim and the Thummim, by saying that they were made by a craftsman from gold and silver, and he continued his discussion of them in this vein, for he thought that they were akin to the forms which the astrologers make in order to know the thoughts of the one who comes to ask of them [about the future]. But what [Ibn Ezra] said is of no import. Rather, the Urim and the Thummim are as Rashi has written...

The Rashba - Shlomo the son of Avraham the son of Aderet, was a leader in Barcelona in the year 5040 (1280 CE), and he was extremely learned in all types of knowledge. And in his time they banned the study of all secular subjects, with the exception of medicine.

Rabbi Hai Gaon was strict about this, and this is what he wrote: Regarding your question about one who took a prayer book, or any other holy book and swore an oath on that book, whether that oath is just as binding as one sworn on a Torah scroll, and if he regrets it and wishes to go back on it, if it can be annulled or not. It has been shown to us from the heavens that this oath is like an oath on a Torah scroll - Just as a Torah scroll contains the names of G-d, prayer books and other holy books also contains the names of G-d, as well as verses from the Torah. Furthermore, because each letter of the Aleph Beis is written in them, the letters of the Aleph Beis combine to form many [Divine] names and many essentia principles of Torah, and the oath cannot be annulled.