**Samaritans were Babylonians who replaced the 10 northern tribes in the Assyrian deportations.**
Babylonians, and others from the Assyrian empire did indeed replace the 10 tribes. The replacement wasn’t 100% complete for one reason or another. Maybe there were some that remained as slaves, or maybe there were some that trickled back. All I know is that Hezekiah sent letters (2Chron. 30)to Ephraim and Mannaseh, inviting the remnant of them “that are escaped out of the hand of the kings of Assyria”(vs 6)to keep the passover in Jerusalem. There is mention of a ‘multitude’ of them coming, also including members of Issachar and Zebulun. Near the end of the chapter (vs 25) they are mentioned again: “..and all the congregation that came out of Israel, and the strangers that came out of the land of Israel..”.
Yes, the Babylonian replacements were instructed in the ways of the Lord by priests sent back to Israel. But, the testamony of 2Chron. 30 seems to show a remnant. In the following 7 centuries, no doubt more trickled back as empire replaced empire. And over that long period of time there was certainly mixed marriages, making them (in the eyes of the Jews) unclean. Of course many were scattered elsewhere. After all, Paul, on his missions, entered long established Jewish congregations to preach the gospel.
In short, I don’t subscribe to ‘lost tribes’ mindset. They were scattered, but not lost.
God knows where they're at even if they don't.