Bisq is probably the most decentralized exchange but it is not truly/fully decentralized.
In a fully decentralized exchange (just like a fully decentralized cryptocurrency) there is no server to connect to and there is no middle man who receives exchange fees (Bisq doesn't check these boxes).
Instead everything happens 100% peer to peer where traders connect to each other and execute scripts (smart contracts) that ensure their safe trade and the only fees they end up paying are the transaction fees (whether it is on-chain or on second layer like lightning network).