Earlier quoted context omitted.
People of that calibre are quite rare but they do exist. I've seen final year undergraduate courses pulled off without the lecturer once having to refer to notes. All proofs were completed in exacting detail on the whiteboard.
Sure, but those math profs have probably taught the same course 20+ times. Going through material that many times will permanently burn it into your brain.
They can talk about their chosen topic at many levels to many different audiences, from general audience (who may provide funding to them), high school students (outreach and recruiting), university students, and peers. This flexibility is an important part of being a very successful mathematician, and you have to burn it into your brain to reach that level of fluency.