You got that backward, Werder loaned him in 2019, first for U19 (19/20) and the second team (20/21), and this year bought him, apparently for the first team. He essentially has been with Werder for two years already.
Remember Son Jun-ho who got unfairly detained at the airport and arrested by the Chinese government? He got freed and arrived at Korea today. China didn't have any proof against Son and so he got freed. It's highly suspicious how long they kept him.