The Sims 4's Doctor Career comes via the Get to Work Expansion Pack and is one of the most challenging Careers I've ever covered.
In this Career, you will start with janitorial duties, interruption of actual Doctors with socials, and patient relations and proceed to being able to run your own tests and diagnose patients. As you go forward, the Career gets more defined although menial tasks will still pop up as objectives. You will eventually be able to perform surgeries and deliver Sim Babies as you advance, while your Sim will also have a clearer picture of what is wrong with the patient as they progress. The Doctor's Career level determines success rate with many interactions. Try to take care of patients as much as possible, and do not misdiagnose them, for fear of losing job performance! This guide should help you with those things, as I have a list of symptoms of each illness, while I'll also share the Doctor's career schedule and rewards.