I connect to a website on port 3000 like so :
curl localhost:3000
a random port is used to start the connection (known as ephemeral port)
you can see this with lsof for exemple :
TCP localhost:46862->localhost:3000 (ESTABLISHED)
ephemeral ports can be tweaked from this file :
cat /proc/sys/net/ipv4/ip_local_port_range 32768 60999