exactly what happens in the Operating System between a user application making a system call to read sector 50

Question

from disk and the data being returned to the user's buffer. You should indicate any process scheduling that occurs; changes in processor state, memory mapping changes, the use of interrupt service routines and anything else you think is relevant.

Answer 1

In any sane operating system, the user application has no way of knowing where any data is stored, so it shouldn't be able to even ask for sector 50.