I just went on Nexo to swap BTC for UST and it says the fee is $0 for me. I'm not sure if that's because I am at Platinum level, or if it is no fees across the board. I have seen in other centralized exchanges like this that they do $0 fees to purchase or swap. But where they make the money from is the "spread" or the difference between the buy/sell price at the time of the transaction. They also, of course, make money from interest and lending from collateral that users deposit into Nexo. Sorry I could not be more helpful! Thank you for reading!