I got a B in quantum physics. There were three of us in the class. Quantum physics was not a popular course back then. A, B, C was the end of term grade distribution.
Quantum math is really quite simple, but the physics, understanding the bizarre world of quanta, is hard to grasp... even by those of us who have spent years in the field. There are effects we know about but, even now, we have no clue how they work. Quantum entanglement is a prime example If you were to explain how that works, you'd win the Nobel in Physics.
I found other aspects of physics much more difficult from a math POV. I took a course in Maxwell's equations for example where the prof handed the final exam out on the first day of class. We had all term to work it. It was take home, open books and notes, and collaborative... meaning we could team up and work the exam as groups. At the end of the term, my team got a C in the exam.
Then, on the other hand, doctoral level physical chemistry was a breeze. I ended up with an A for that course and didn't even have a textbook. I just studied from my lecture notes.
Depending on how the school runs the doctoral program, you'll need to take some sort of comprehensives. In my school, that meant written comprehensives that lasted two full days and covered every course I took at the PhD level. As my classmates were also taking the comprehensives at the same time, we teamed up and studied as a group. We met on the weekends at the school.
At each meeting, we'd go over one of the twelve courses we took, including electives. One candidate would present the material he/she thought was important to know. Then we'd discuss it, review it, etc. until we were ready to move on to the next subject. We met for about six weekends, both days each, to cover all the material. The good news is that everyone in our study group passed them.
The bad news was that only two of us actually went on to get our PhDs. The roadblock was the dissertation. It took me six years to complete my dissertation because I worked on it part time. I had a full time job as a rocket scientist for Lockheed Martin; so there was little time and energy left to work on the dissertation. But I plodded away and finally earned the PhD. Total time from taking my first doctoral course until receiving the degree.... eight years. Whew.