Besides some of the very, very obvious (don’t copy/paste 100 lines of code, make it a function! Write comments for your future self who has forgotten this codebase 3 years from now!), I’m not sure how to write clean, efficient code that follows good practices.
In other words, I’m always privating my repos because I’m not sure if I’m doing some horrible beginner inefficiency/bad practice where I should be embarrassed for having written it, let alone for letting other people see it. Aside from https://refactoring.guru, where should I be learning and what should I be learning?
The most important thing is that the codebase can grow without too much refactoring. Then you know you got the overarching design right. The rest then doesn’t really matter that much. You can always rewrite certain parts when/if needed.
A good way to do this is by making the core really solid, this is called bottom up programming: https://paulgraham.com/progbot.html