I have become fairly proficient with OnShape thanks to their free courses and while I am fairly competent with the controls and achieving something serviceable, I would really love to get into learning more about CAD/product design best practices.

There's probably a class or two in freshman engineering that gets into this stuff but I am mostly picking things up through trial and error. I am mostly just poorly imitating stuff I have seen after a few decades of taking things apart and occasionally putting them back together.

Like: How should I design two parts to fit together optimally? How should I decide what kind of hardware and why, or where? Screws? Bolts? Glue? Holes, where and why? What are some things to look out for when designing universal parts?

Also, are there any good references for working with PLA? I have a good sense of what things will look like after slicing and have got pretty good at making things that will print well and be strong but I could always use more references.