I was reffering to the quality of l2 players and their expectations, since everyone can open easily a server with the LATEST release, but cannot improve their own server or even take the time to set it up decently, the l2 community is flooded with tons of questionable private servers, like mushrooms, of really bad quality in terms of management, setup, world creation (corrupt childish gms, server wipes to get fast donations, and other shit) that makes possible l2 players turn away from the game.
It`s not about the quality of mobius, but the share amount of private servers ran by children. You are handed a great tool, and most use it for fast gains at the detriment of losing l2 players globally. Untill mobius, there were gatekeepers to get decent builds, now they are not.
If anything this speaks of the quality of l2 mobius, and not against it.
Not everything is an attack, things are more sublime in the real world and l2 is dying, having tens of thousands of shitty private servers is not helping.
This is my last reply, seeing the answer of the creator of that build, again, for me it feels like children running around with bazookas.
Better use an active fork or L2J itself and join their community (generally, each project got at least a Discord) - you will generally find dedicated ppl to answer your questions.
About practicing, as I said, you have to at least understand Java OOP concepts (mandatory, will help you a lot to understand what you exactly do - if not, you will eventually guess it after 2-3 months).
You should apply manually customs (adapting them from one project to another is even better, since it forces you to seek through files). Start with basic customs, few lines max. Then raise the difficulty with more advanced customs.
In the meantime, try to modify them for your needs or experiment.
It's really annoying and hard to apprehend for a newbie, but you should also read and understand about git/svn (version control systems). Those systems allow you to generate "restore points" everytime you commit in your project, allowing you to eventually revert back. It will be extremely useful once you do crap. Gitlab / github (GIT technology) is the way to go those days, but SVN is really fine (and easier to use, since more basic) if you work alone. Try even with a non Java project, it's not important, from the moment you understand how to generate such a repository, commit over it, and be able to revert back.
If you don't use such system, your project will eventually break one day, and you won't be able to revert what you did. That's basically how I lost my first Java project, back in 2010.