Hot Take: there should be no Q&A. Best-case scenario, the questioner uncovers some omission that the speaker tweets out later. Average-case is irrelevant questions or some super-specific case that makes the speaker question if they can bill consulting hours to the questioner. Worst-case is giving a self-aggrandizing questioner a microphone to attempt to upstage the speaker. None of these are a benefit to the audience except in the first (uncommon) case.
Just go find the speaker later and ask questions if you have them.