im not sure about the clothes, but from my own speculation it could be the fact that
1) yeonwoo felt comfortable enough to give Jaeeon his washed and fresh clothes, despite being a hated presence of Rosa. Like it shows almost a closeness that was never previously mentioned. Also the very fact that Yeonwoo gave those clothes back to him infers that yeonwoo is aware that jaeeon and rosa slept together, which is not something that he should have been privy to given the relationship that Jaeeon thought rosa and yeonwoo shared (one of hatred/contempt)
2) i think the expression was just one of jealousy/possessiveness that otherwise is just not normal to have for someone ur not intimate with
Excuse my English, but I think it's because you usually tend to keep the clothes of the guy you're seeing, to wear them at home and have his scent, haha. The fact that Yeonwoo washed them to get rid of his smell and looked at him with an unfriendly expression made him realize that he wants him away from Rosa, and I suppose that because he knows Rosa and how she's been acting lately, he came to that conclusion
Why did Yeonwoo giving him the washed clothes make Jaeeon figure everything out?? They were washed like he said, how did it make it click for him so fast... Like I feel normally your mind would reject such an idea... And what was Yeonwoo's expression that helped make things click? Did he look jealous or annoyed? I couldn't figure it out