How does one remove cores as a player in the game?

[NatRed 0]

I've seen Catalonia removing the cores of Spain in one run. But I didnt take a photo.

I have been trying to replicate it myself but have since failed.

[PANTIC_TIME_NEWS 84]

Catalonia will not yield😂😂🇪🇸🇪🇦

[Federation of Arizona 110]

if you still havent fixed this, (i don't know either but this is a guess) you probably need to invest a ton and assimilate several times, or just try to anything you can to lower the original countries population in the province. (again, this is a guess so it might not work)