Yeah, basically after the USSR invaded the Baltic, Stalin deported loads of locals, and moved loads of Russian people in instead, because he was Stalin.
Later on, the Baltics had a much better standard of living than other parts of the USSR, and didn’t have the high competition to get a place in the housing list like Moscow or St Petersburg, so lots of people moved there voluntarily. Russian was used as the official language, much to the chagrin of the Estonians, Lithuanians and Latvians.
When the USSR split up, Estonia and Latvia introduced language tests to get a passport if you weren’t local. There were plenty of people who had lived in the area nearly all their life, but didn’t actually speak the language, because you could do everything in Russian. And Russia did some dick moves with passports too, and it’s been this whole grudge match for the last 20 years or so.