You are asking for the actual nuts and bolts? Everything is accomplished over telegram in the crypto space, including OTC trades. Circle had entities throughout the world and offshore bank accounts with those entities. You would ping Tether on telegram and say you want to swap USD for USDT or vice versa and wire transfer details would happen. It isn’t rocket science. I don’t know the exact amount but we had pretty transparent internal disclosure of the business and what each unit made in profit / loss, and what he said rang true for me. Circle Trade did something like $25 billion in OTC trades in 2018.