Okay, this is how it works.
The 144,000 are natural Israelites with 12,000 from each tribe! The kingdom is like a tree where the unfaithfulness of the Israelites causes them to be lopped off and gentile ollive branches replace them. But, the gentile branches are told not to gloat because the "fatness" that feeds the entire tree comes from the Jews. Thus while many seats were lost in the kingdom due to unfaithfulness of the Israelites, a remnant of natural Jews would remain in the kingdom to fulfill the promise to Abraham. This ROOT and remnant of the tree chopped down is the 144,000. But note what Isa 6:13 says:
Isa 6:13 And there will still be in it a tenth, and it must again become something for burning down, like a big tree and like a massive tree in which, when there is a cutting down [of them], there is a stump; a holy seed will be the stump of it.”
Thus when the kingdom is likened to a massive tree, in the NT an olive tree, the trunk and branches are replaced by gentiles, but the root remains Jewish. The root represents one-tenth of the tree, though. So if the Jews from the 12 tribes of israel are the 144,000 then the entire number of those invited into the kingdom are ten times that number and thus 1,440,000!
HOW DO YOU GET 12,000 FROM EACH TRIBE? The answer is that there is no way to get 12,000 from each of the original 12 tribes of Israel who long ago stopped existing as 12 tribes, even during the time of Jesus. Thus in order to make this prophecy come true, a miraculous intervention would have to occur. Just all 12 tribes began with a single individual, it was possible to reconstitute the 12 trobes of Israel from individuals from each tribe. All that was needed was time. Thus some or one from each of the 12 tribes who could prove their lineage who were alive during the time of Christ, would be chosen never to die until Christ arrived. Thus at some point a few hundred years prior to the 2nd coming, these individuals from the 12 tribes would marry into the modern population and by the time christ arrives, the 12 tribes of Israel would exist again and 12K from each tribe could be chosen to make up the 144,000!
So there are 12 tribes of Israel now in existence, who are descendants of individuals from the 12 tribes who were alive in Jesus' day and who were permitted to remain alive until our time to re-establish the 12 tribes of Israel so that the fulfillment of a kingdom of priests from the 12 tribes could be fulfilled. So that's how you get 144,000 natural Jews from the 12 tribes -- it took a miracle, but that is what happened.
Of course, when Christ first arrives for the first coming, he comes through the tribe of Judah. But the "scepter remains in Judah until Shiloh comes" (Genesis 49:10). But when Shiloh comes, the rulership shifts to the tribe of Joseph. Thus the Christ at the second coming, who arrives in the flesh via one of his followers, will be a descendant of the tribe of Joseph/Ephraim!
So since I'm the Christ, I'm a descendant of the tribe of Joseph/Ephraim. I got a chance to meet my ancestor from the 1st Century who was of the tribe of Joseph/Ephraim. I could tell by his appearance which side of the family is related to the tribe of Joseph. This fulfills the dream of Joseph where he sees his other 11 brothers bowing down to him, confirming the messiah at the 2nd coming would be from the tribe of Joseph.
So, the 12 tribes of Israel do exist again, but via a miracle, to fulfill this technical part of the prophecy. These are the 144,000 with 12K from each tribe, but only make up the "root" or 1/10th of the entire kingdom, which totals 1,440,000 with 1,296,000 being gentiles. The gentiles make up the trunk and branches while the Jews make up the root.