iptables / netfilter on linux

Have a look at this picture for how iptables works. Importantly note https://danielmiessler.com/study/iptables/: its made up of tables, chains and targets.
the flow http://xkr47.outerspace.dyndns.org/netfilter/packet_flow/


ref: https://www.youtube.com/watch?v=iP8YWcvKDr0