Charles Ellicott Commentary Leviticus 8:8

"And he placed the breastplate upon him: and in the breastplate he put the Urim and the Thummim." — Leviticus 8:8 (ASV)

And he put the breast-plate upon him. —Called more fully, “the breast-plate of judgment,” which was also a distinctive pontifical garment, and which was made of the same costly materials and the same skillful work as the ephod. (See Exodus 28:15 and following)

Also he put in the breast-plate the Urim and the Thummim. —Better, and he put into, etc. , that is, Moses put into the bag of the breast-plate these material objects which were separate from the breast-plate, as well as from the gems set in the breast-plate. (See Exodus 28:30.)