A trick for learning things you don’t know is to find a marketplace asset for a system you want to build. Generally most systems are robust as they account for features that would appeal to all game demographics that use that system. You can go through their code, read their documentation, figure out how it all works. Then build the system you want in your game and customize it with only the features you need.
There are free projects of nearly every type that you can break open and study. If you need a better understanding with help, purchasing an asset with an active developer might be better. All the ones I have talked to have been very helpful and will try their best to help you get their code to work in your project.
Also keep in mind, game development is a massive field of skill-sets. It requires patience, dedication, thick skin, and a ton of luck to make it in the indie and even professional world. If it was as easy as flipping a switch to turn on online mode and multiplayer just works, we would see a ton of MMORPGs and other types of multiplayer games taking over the multiplayer market. However as you can see its mainly AAA companies with massive budgets and teams. A few lucky indie companies too that played their cards right, but that’s it.
Someone wise once said, “If it was easy to do, everyone would be doing it.”
If this is truly a path you want to follow, don’t give up, stay motivated and remember you will make mistakes, it will take a long time. Good luck