This chapter has to do with the consecration of the priests and of the Tabernacle. When it says “all the congregation,” it means the entire nation was to assemble there. The Hebrew word for “congregation” is edah.
Since the convenant has been re-instituted, the Tabernacle may now be built. The instructions were given in earlier chapters, but the construction had not yet taken place.
What is the Urim and the Thummim? Most scholars say the words stand for “lights and perfections” (or “true instruction”). They were kept in the breastplate and apparently Elohim used them to give the High Priest “yes” or “no” answers.
Elohim is not going to restrict His presence to this one structure, but He would dwell there. His dwelling place would specifically be between the cherubim on the mercy seat (2 Sam. 6:2). The Mercy Seat is the lid to the Ark of the Covenant.