Who are those referred to as "my people?" "My people" are "the remnant of Jacob."
Who are those referred to as "they?" The "they" are gentiles who have repented, come to Christ, entered into a covenant with Him, received the fullness of His Gospel, become heirs, and received the promise of land, and a connection with the promises to the remnant.
So it will be these few, chosen, covenantal gentiles who will "ASSIST" the remnant.
-They won't lead them,
They will "assist" them. What does "assist" mean? Who is taking the lead if the gentiles are only to "assist" in the process?
What will the remnant do? What city is to be built? Why is it called the New Jerusalem?
Forget everything you think you know about where the New Jerusalem is to be built. Most of the myth and traditions about it are based on incomplete and inaccurate recreations of the events.
Joseph sent the first missionaries to the Lamanites to find the place. The entire block of Native Americans east of the Mississippi, from the Delaware to the Cherokee, had been relocated at the time of the 1834 revelations regarding the New Jerusalem. At that brief moment in time, all of them were located just over the boundary of western Missouri. The closest you could get to them was Independence, Missouri. Since it was the remnant who would build the New Jerusalem, the obligation was to find them, preach to them, and assist them in building. But the missionaries couldn't do that. When they tried, they were sent out of the Indian Territory on the threat of being imprisoned. So Independence was as close as they could get.
The Native Americans have relocated and relocated again. Now they are nowhere near Independence. When Joseph fled Nauvoo in late June, days before his death, he was leaving for the Rocky Mountains where he intended to locate the remnant. He returned, was killed, and never made it out here.
Brigham Young tried to locate the remnant. In fact, the St. George Temple was built as the next fully functioning Temple at the chosen location precisely because it was intended to be near the remnant. In the very first endowment session, the Hopi Chief and his wife went through, received their endowment, and were sealed the next day. They were invited to try and connect with the remnant and this tribe was suspected as the one the Saints were to locate.
We've lost that fervor. We've assumed Independence is the site. We think we're going to build it. We have no clue we are only to "assist" and not control.
All of this is worth some study. But you're going to have to search back into history and ignore all the recent re-done and re-worked histories that ignore this early material. It's too much to get into in this post, but maybe I'll take it up at some point.