I have time for a brief answer:
There is one people of God made up of Jews and Gentiles. And when the Lord returns and rules this world, he will rule over Israel and the Gentile nations (Ezek 36:22–38). The millennial kingdom of God—and this is important—will not be nationless and ethnicless. Those in the new covenant of God will make up the one people of God, as they do today. In the millennium the promises of God to national Israel will be fulfilled. Israel will be the prominent nation among the Gentile nations.
There are two extreme errors of thinking. Classical dispensationalism have believed that there are two peoples of God—Israel and the church, even some of them separating the church in heaven from Israel on earth forever! The other extreme is replacement theology which says that God’s promises in the Old Testament for a national redeemed Israel at the consummation have become abrogated (or replaced) and are now fulfilled in the church.
The Bible teaches, instead, that the one people of God in the millennial kingdom will be diverse in ethnic, national, and social aspects—it will not be a monochrome kingdom.