v6 API uses JUP6LkbZbjS1jKKwapdHNy74zcZ3tLUZoi5QNyVTaV4 with versioned transactions & address lookup tables so all swap routes take just 1 transaction. Also, no intermediate token accounts and open order accounts are needed anymore.
v4 API uses JUP4Fb2cqiRUcaTHdrPC8h2gNsA2ETXiPDD33WcGuJB with versioned transactions & address lookup tables so all swap routes take just 1 transaction.
v3 API uses JUP4Fb2cqiRUcaTHdrPC8h2gNsA2ETXiPDD33WcGuJB which uses legacy transactions so some swap routes can take up to 3 transactions.