ㄴ가 들어가다

something gets in; go into somewhere

The -ㄴ가 here belongs to a vague word rather than to a verb ending. With 뭔가 it is 'something gets in' — 침대에 균이 뭔가 들어갈 것 같은 거예요, 'it feels like some germ is getting into the bed'. With 어딘가 or 어디론가 it is going into some unspecified place: 진짜 산골 어디론가 들어갈 줄 알았지. After a number, -인가 makes it an estimate: 6만 갠가 들어가요, 'about sixty thousand of them go in'.

Register: casual spoken