> I commonly see ceiling exhaust fans in bathroom showers and toilets with windows sealed shut. The door is meant to be closed... so the only gap for air to come into the room is around the door somehow

If the building has central air heating or cooling, having a vent into the bathroom is standard. That’s more than enough for the exhaust fan to pull from.

In areas where central air is uncommon, you need a decent door undercut. A surprising amount of air can flow around a door and modern exhaust fans can pull a relatively good vacuum.