How does a firewall (Firewall) open a port and how is this process technically controlled?

Question

firewall

please explain how it works

Answer 1

> as a firewall (Firewall) opens the port
opens - in general, the wrong concept. After all, the OS opens the port at the request of an application that wants to listen to it. and the firewall generally filters the port: allows or denies traffic on it (or part of the traffic).
the firewall checks its configuration, and if it says that traffic to the specified port should be allowed, then it does not prevent its passage.
> how this process is technically controlled
on the processor, a code is executed that, on the software and hardware platform used, is able to prevent traffic from passing to the ports / addresses / protocols specified in the configuration, etc.

Answer 2

And how is this process technically controlled?

Depending on what level you need an answer. At one level it looks like this: