Ask HN: What are some good resources for coding best practices?

I've been recently promoted and am now managing a summer software engineering internship. Based on the feedback we got from the interns last year, I want to teach them best practices and good principles for writing good code and designing maintainable solutions.

They've heard about DRY and SOLID, we're writing unit tests, they're familiar with git and we're applying agile principles, but I'd love to hear what you all found helpful early in your career or what resources you use now to teach junior engineers the fundamentals.

Thank you!