Based on the Ramban and Rabeinu Bachye, the Urim ve'Tumim were Names of Hash-m (the Ba'al ha'Turim refers to the seventy-two letter Name of Hash-m, and the Targum Yonasan, to the Name with which Hash-m created Heaven and Earth). Presumably written on parchment, they were placed inside the folds of the Choshen, which was specially made double and folded for this purpose. The reason in fact, that the Torah declines to describe them, is because of their Divine Character. Moshe was taught about them and how to prepare them discreetly, it seems, and nothing was publicized.

As you comment, they encompassed a specific level of Ru'ach ha'Kodesh, lower that that of Nevu'ah, but higher than Bas Kol, which was still used in the second Beis-ha'Mikdash, when Ru'ach ha'Kodesh was already disfunctional.

Here is the Rambam's description of how they were used (Hilchos K'lei Mikdash 10:11:12): The Kohen (Gadol) stood facing the Aron, and the questioner, who stood behind him but facing in his direction, would then ask (for example) 'Shall I go up or not'? He would ask neither in a loud voice, nor would he just think his question, but he posed it, vocally but quietly, to himself, as one Davens the Amidah.

Immediately, Ru'ach ha'Kodesh would envelop the Kohen, and he would look at the Choshen and perceive the answer 'Go up!' or 'Don't go up!' in the letters (that were engraved on the stones of the Choshen and that now protruded from them before his eyes. Whatever the answer, he would pass it on to the questioner.

One only asks the Urim ve'Tumim on behalf of a king, Beis-Din or someone whom the community needs, says the Rambam (and he gives as an example, the Kohen Gadol for war).

You remark that they seem to have been consulted mainly in the context of battle. This is not surprising, as the Gemara in B'rachos 3b. implies that they would not go to war without first consulting the Urim ve'Tumim.

There is a famous explanation of the G'ra, according to which Eli ha'Kohen consulted the Urim ve'Tumim about Chanah, whose lips he saw moving in prayer but no sound coming from her mouth. In response, the letters 'Kesheirah' or 'ke'Sarah' lit up (according to the Rambam, they would have protruded), but Eli misread the letters as 'Shikorah'. I am not sure in what capacity Eli consulted the Urim ve'Tumim for something so trivial, but in any event, it wasn't for war.

Finally, you ask why it was necessary to have the Urim ve'Tumim and Nevi'im simultaneously. I suggest that a. the Navi's main function was not to answer questions (the same Gemara in B'rachos says nothing about consulting the Navi before going to war), though he may have done that as well, whereas the Urim ve'Tumim's was; and b. the Urim ve'Tumim was meant to enhance the prestige of the Kohen Gadol, which will explain its being part of the Bigdei Kehudah. It may well also be because the Nevi'im were often sent on errands, and were not always available to answer question; the Kohen Gadol on the other hand, was always on hand.

Incidentally, according to the Rambam (Halachah 10), the Urim ve'Tumim existed in the second Beis ha'Mikdash, in order to complement the eight Bigdei Kehunah, only it was not used because Ru'ach ha'Kodesh was missing during that period.

Be'Virchas Kol Tuv

Eliezer Chrysler.

Thank you. The Vilna Gaon adds that many of the Bnei Nevi'im or Kohanim would wear an Efod Bad, which sometimes provided them with correct answers through Nevu'ah.

MK