How are mathematics conference presentations usually done and what to expect from the question and answer component?
I'm not sure I really understand what you want to know.  Have you never seen a conference talk? (If you haven't, you can watch some here.)  Perhaps you're going to different conferences from me.  The question and answer part is usually 2 minutes at the end of the talk; occasionally no one has a question, but often to be polite people will ask for a clarification of some point or ask about a connection to something they free-associated about during your talk.  You should do your best to answer, but people will understand if you say "Gee, I hadn't thought about it that way" or "Let's discuss that during the break."  Generally people are eager to get to lunch/coffee break and won't draw it out too long.