Students choose only one of the Cx courses.
Then there are PhDs corresponding to each of them: www.ox.ac.uk/admissions/graduate/courses/mpls/physics
Video 1. A very honest review of my Oxford University master's degree (theoretical physics at keble college) by alicedoesphysics (2020) Source. Basically all her courses are from the Mathematical Institute of the University of Oxford, and therefore show up at the Moodle of the Oxford Mathematics Institute of Oxford.
Interesting presentation cycle at Merton BTW: www.arturekert.org/teaching/merton
Artur looks like a cool teacher.