Textbook vs Real
Textbook게임이 끝났습니다ge-im-i kkeut-nat-seum-ni-da
Real게임이 끝났다ge-im-i kkeut-nat-da
Why Koreans say it this way
Plain form ~다 for dramatic finality. When something is truly over, Koreans drop to plain form. 끝났습니다 sounds like an announcement; 끝났다 sounds like fate.
From: 오징어 게임 EP6