Github repositories recommend for backend newbie [on hold]

I am about to graduate and begin my career as a back-end engineer (at least for first 2 years I think) soon. I major in Computer Science, and I think I did OK in my study in terms of school stuff. But I feel worried about the coming work, you know, I never developed a whole system focus on the backend. I mean I did some complete projects, like websites and smart phone app, but I basically use the APIs of other mature tools. But I think as a back-end engineer, I must be able to create these tools, but not only use them, right? I think reading the good source code will be a good way to learn. So, my question is could you recommend some opensource projects that good for a fresh back-end engineer? I hope the recommendations can be sorted by difficulty so that I can read one after another.

Thank you.