If you're asking "where is the listener", you don't need one: https://datatracker.ietf.org/doc/html/rfc9293#simul_connect

RFCs may say that simultaneous connect must be allowed, but that doesn't mean that firewalls can't block it. Plenty of setups block incoming SYN,!ACK packets, and if both sides do that, the connection is never getting established.