The simple setup will suffice for most users. The only reason you would have to deal with any firewall it would be your router firewall to enable port forwarding for a vpn and such. Maybe dedicate forwarding to improve a game system. How you use your system and what you do with it, a rudimentary understanding is all that's necessary. If you really want to dive into it... here is a link to MIT basic configuration principals.
http://web.mit.edu/rhel-doc/4/RH-DOCS/rhel-sag-en-4/ch-basic-firewall.html
As for grasping it, it took two months of structured learning, a love for networking and constant practice to understand the basics for me (i learn quickly and have a passion for the topic). Unless you too have a passion for it and can absorb dry technical jargon like a sponge, the same two classes would take you from 3 to 6 months and still will barely grasp the basics. I applaud your enthusiasm for wanting to learn it, but you soon realize after all that time, the default is fine.
Since you're not doing pen testing or illegal hacking, it should be more than fine for you. The following is a link describing the very firewall you are asking about. The only thing i ask is that you don't change anything you don't understand, for your protection.
https://linuxconfig.org/how-to-configure-firewall-in-ubuntu-18-04
I am not trying to be condescending or rude, in my experience people just learning something tend to think they can do it better, and for the most part they are wrong. Firewalls have been around for over 2 decades with the world's sharpest minds working on them to improve security for everyone.
Do not open anything to the internet that you don't want accessed. It may be fine for a while, until you find there is someone lurking in your network, watching your ring doorbell and rerouting emails and messages until your identity is finally stolen.
Learn the ports and what they do before opening them. Ssh can be a big deal, especially improperly configured (and an eight character password is not secure). Ipsec and keys are the best way, but those need configured too. And they are they're own topics to research.
It will get complicated fast and require more study of other topics along the way.
Understand the basic part before attempting the advanced. Most of what you think will help is already applied in the basic setup.
Happy learning



