What if the question is born out of the insight that his team's software is poorly designed, motivating the desire to get outside input?

What if the question is asked by a college student?

Then it still matters a lot what kind of codebase he is working on. A web project is differently structured and done than the codebase for a embedded operating system. There are differend standards and practices in the industry.

I learned a lot by just stepping through the code with the debugger of libaries I used. That brought more practical insight while learning about design patterns etc. In the end, it is all about patterns. Finding the right pattern for a given problem.

I would be interested in recommendations for those who have a poor example or no examples

In my case, I'm a junior engineer that has recently been given more responsibility designing aspects of our product. I'm just trying to learn all I can so my designs will be good!

On forums like this people have a frustrating inability to answer the question asked.