Your shift ended at five. Answering at nine teaches them you’re always on the clock.

Whatever you answer sets what is expected of you, quietly and quickly. Reply at nine at night once and it is kind, and reply every night and it becomes the baseline, with the next request assuming it. Nobody will tell you this is happening, because from their side you simply seem helpful. The boundary is set by your behaviour, not by the schedule on the wall.

If a message comes in well after your shift, leave it until the morning and answer it then, warmly. No explanation needed. If something is genuinely urgent at a job that pays by the hour, that is theirs to staff.