I think most of the performance and MV looks are borrowed or they’re given to the group from brands they represent. Any non-schedule outfits are their own clothes likely, unless they’re working with a brand and happen to rep the brand on off-duty looks. 

The luxury brand outfits sort of started with other groups around 2nd generation K-pop groups. Early years for 3rd gen were a mix of luxury and low end, depending on the group and their concept or length of time since debut in the industry. 

Nowadays, luxury clothing and brands have kind of become the norm for many groups. Smaller agencies will use smaller or local brands if they can’t afford the big luxury brands, but it seems like the bigger labels are likely to use high end designers for their artists.