You don’t need to be an experienced public speaker or debater to do well in the interview. The focus of the interview isn’t to assess whether you’re charismatic, but rather to determine whether you have the traits and qualities of a good medical student and a good medical professional.
(Having said this, you do need to be able to express yourself well enough to show that you have those traits!)
Different medical and dental specialties appeal to people with different personalities because they have their own characteristics. Not all doctors and dentists need to be charismatic or confident speakers: some specialties are more people-facing, while others are less people-facing (e.g. radiology), and the types of interactions required in each specialty also vary.
Public speaking and debating do help with things like nerves, but the skills are not as transferable as people often think because the purpose of public speaking and debating is very different to the interview. In fact, candidates who treat the interview as a speech or a debate will likely find themselves in a lot of trouble during the interview.