Retainers are usually needed afterwards, so raise it at planning. The Dental treatment guide describes a retainer as part of maintaining tooth position after orthodontic care. Ask which type is proposed, whether it is worn at night only or all the time, and when that will be confirmed. Braces and retainers guide describes both fixed retainers behind the teeth and removable ones, so a friend's instructions may not match yours.
Before your last aligner, ask for the wearing, cleaning and review instructions in writing. What should you do if the retainer is lost, damaged or stops fitting, and who do you call? Use the fees page to prepare questions about retention costs: are fitting, reviews, repairs and replacements included, and what would be charged separately? If you later move or change dentists, ask how your records and follow-up information would be passed on. Finishing the aligners is the point to start that conversation, not to leave it open. A clear plan for retainers makes the end of treatment easier to manage.