Simple:
"Behold, the days are coming, declares the Lord, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the Lord. For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people."
-- Jeremiah 31:31-33.
By specifically referencing the "house of Judah and the house of Israel," the prophet means to identify the entirety of the Israelite nation that was formed under the old Sinaitic Covenant -- i.e., all 12 tribes (well, 13, really, when you include Levi). But, of course, at the time of the prophet Jeremiah's communicating this message from Yahweh/God, the 10 northern tribes no longer existed. Having been carried into exile by the Assyrians in the 8th century B.C., they were to be dispersed throughout the Gentile nations, among whom they eventually intermarried and lost their character as "pure" Israelites under the Sinaitic Covenant.
Flash forward to the Last Supper:
"And he took bread, and when he had given thanks, he broke it and gave it to them, saying, 'This is my body, which is given for you. Do this in remembrance of me.' And likewise the cup after they had eaten, saying, 'This cup that is poured out for you is the new covenant in my blood.'"
-- Luke 22:19-20.
The New Covenant announced by the prophet Jeremiah was promised by Yahweh/God to the totality of the Israelite nation . . . even though, at the time of Jeremiah's announcement, the northern portion of that nation no longer existed. This New Covenant was inaugurated by Jesus of Nazareth at the Last Supper. The regathering of all the tribes of Israel, including those descendants of the northern kingdom that was dispersed among the Gentile nations, began at Pentecost:
"Now there were dwelling in Jerusalem Jews, devout men from every nation under heaven. And at this sound the multitude came together, and they were bewildered, because each one was hearing them speak in his own language. And they were amazed and astonished, saying, 'Are not all these who are speaking Galileans? And how is it that we hear, each of us in his own native language? Parthians and Medes and Elamites and residents of Mesopotamia, Judea and Cappadocia, Pontus and Asia, Phrygia and Pamphylia, Egypt and the parts of Libya belonging to Cyrene, and visitors from Rome, both Jews and proselytes, Cretans and Arabians—we hear them telling in our own tongues the mighty works of God.” And all were amazed and perplexed, saying to one another, “What does this mean?”
-- Acts 2:5-12.
In sum, the New Covenant is between Yahweh/God and the whole nation of Israel. Those who partake of this New Covenant are those Israelites who are in Christ.
This is pretty much New Testament 101 stuff.