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?
You should try to write code using SOLID principles. You should then write one using CUPID, try following OOP patterns, functional patterns and good old procedural ones. You should develope one huge code base and then try to maintain it for years. You should build a binary in Ubuntu, in alpine, in nixos, in Mac, in Windows and try to ship them to each other. You should run your code on a bare metal server and you should try writing lambdas. You should start a green field project and a try to maintain a legacy system that none of the original authors are there yet. You should write code in a company that hires 500 people per day and lays off in thousands. You should write code for a company that has 2 engineers that have been there for years. You should write a backend, a frontend , and one that does not do all these. And you should ship them. And use them. You should write codes idiomatically and you should write them idiotically.
The idea is not that a good programmer had done all these, the idea is that no simple tip or priciple can apply to every situation. Anyone claiming that, is no more than a snake’s oil salesman. You should learn to code and design and engineer your software critically based on each situation. You should constantly learn. And you should not be afraid to go against the grain or break from the beaten path or go with the most popular mediocre choice.