Hi,
The principals of a firewall are always the same, using pfSense, or some other system, OS, etc.
In the past, and I'm not talking centuries here, the very basic concept of a firewall would take a year or so for a student to learn. And this wasn't on high school or something like that.
These days, we are obliterated by the cheer number of books, Internet discussions, and videos about the subject.
I advise you to start with reading general wiki pages - and have a look at this : https://www.youtube.com/channel/UC3Cq2kjCWM8odzoIzftS04A/videos - there is a video called Firewall and NAT Fundamentals on pfSense that should see.
When you test for yourself, always start as simple as possible. Always check your works.
You can see pfSense as a car. The car builder won't learn you how to drive ^^
Btw : Your first paragraph : you are correct.