Iptables problems ??

Iptables problems ?? -- Linux general technology-Linux technology and application information. For details, refer to the following section. Assume that the gateway is configured with a firewall and the server is on the Intranet.
When the Internet accesses an intranet server, when the requested package enters the gateway, it first goes through the PREROUTING chain and then selects the route. If it is local, it goes through the INPUT chain, if it is an intranet server, it goes through the FORWARD chain and then the POSTROUTING chain to the Intranet server.
When the Intranet server responds to the Internet, does the response packet pass through the gateway through the PREROUTING chain filter? Or directly go to the FORWARD chain for forwarding ???

I personally think that the response packet does not pass through the PREROUTING chain, because when I set the default PREROUTIN chain policy to DROP, the Intranet cannot initiate a connection to the Internet, however, packages that can respond to Internet requests.