Can I ask a follow-up after the answer?
Often yes. Some tiers include a follow-up and the tier description says so before you pay. Beyond what is included, a follow-up is a new paid question.
Use it for clarification
An included follow-up is for "what did you mean by that", not for a second topic. A new topic gets a better answer as its own question.
60 answers in the Nudge help centre.