거 is casual 것 'thing, one', and the modifier says which: 뭐 좋은 거 쓰겠죠 'they'll use something good', 참기름도 되게 좋은 거 쓰셔서 'they use really good sesame oil too', 우리 집 거랑 똑같은 거 쓰네 'you use the same one we have at home'. Note that 쓰다 covers three things across these lines — using something, wearing something on the head or face (가발, 마스크), and writing (편지 같은 거 써놓는 거 'leaving letters like this').