Most of these subjects are not especially well-taught in college CS courses. You're in a golden age for being able to learn whatever you like: you can start by

Most of these subjects are not especially well-taught in college CS courses. You're in a golden age for being able to learn whatever you like: you can start by plugging keywords into YouTube and looking for popular video explanations of how to solve problems in that field, including screen sharing of live tutorials. I especially don't think anyone gets expert-level knowledge of databases and machine learning and operating systems from their CS degree. Consider the possibility that you're actually averagely prepared for a graduate, and this is the part where you mainly need to decide which disciplines actually interest you. G'luck!