It is complicated, but there are centralized exchanges that we all use and decentralized ones that a few of us use. The earliest ones I think were NXT and then Counterparty. You would have to research any of them individually. https://crypto-bridge.org/ https://openledger.info/ https://wavesplatform.com/ (Here are three good ones live now)
RE: Why decentralized exchanges are the future, and my journey through the pure chaos over the years