Israel represents flesh Israel and Judah represents spiritual Israel... is this right?
Israel was all 12 tribes up
until Reheboam, the son of Solomon.
Because God was unhappy with Solomon, the Kingdom was split in two under Reheboam.
The Northern Kingdom was all the tribes, except three. It was called Israel because Ephraim, the son of Joseph, held the birthright.
The Southern Kingdom which was severed from Israel was called Judah. The Levites had already joined with Judah previously, and God left Benjamin with Judah, "to give Judah a light".
So now we have the Kingdom of Israel (9 tribes), and the Kingdom of Judah (+Jerusalem), (3 tribes).
Then Israel was divorced by God owing to her idolatry. In 721BC the Assyrians swept in, and the bulk of the Israelites were scattered. Removed from the land. Where they went is disputatious, but actually not that complicated. History show that some formed the Parthian Empire and eventually migrated into Europe. Some escaped to the Isles (Britain). They surely found a place in many other countries. The Pashtuns in Pakistan have a high incidence of left-handedness and their lore says they were Israelites - Benjaminites in fact (so not all Benjamites necessarily stayed in Judah). The dispersion of Israel was staggered, so the tribes could have settled in different places....
More importantly, Jesus knew who the Israelite tribes were.
When Israel was divorced, the name of Israel was conferred on Judah. God now was effectively working with two different groups of people. When God divorced Israel (the Northern Kingdom) he promised, (in Hosea) to remarry them.
When Jesus came his ministry was to the whole house of Israel. The time had come to remarry Israel.
That is why he said he would give the kingdom to a nation bearing fruits. He meant the Israel of the dispersion. Released from the Law, (remember, the Levites all went to Judah prior to the split), the Israelites freely and gladly accepted Jesus.
When we come to Paul in Romans, he explains all this, but it is somewhat complex, as he does so through OT references, and he calls the process a mystery, and to cap it all, he frequently refers to the Jews as Israelites....