How to increase Code Quality & testing as a Freelancer

I'm a self-thought dev and I started learning to code around 2 years ago. I'm already comfortable with documentations, many programming concepts. But I feel lacking in code quality & testing. When I look at some other projects it seems like their code is so simple with less lines, but does the same things that I do in more lines of code.

I'm mostly doing nodejs + reactjs. I took some software construction courses online, but they were mostly talk of concepts with very few examples of them. I'm also not working in a company so getting mentorship from a colleague is not an option.

How do you guys think that a freelancer can increase their code quality & testing?

Where can I learn the best coding practices?

Can you offer me any good open-source projects with high code quality and standards? or some books to dive into?

I appreciate every advice.