Not a regular occurance, in my experience, or even frequent!
Once disembarkation has begun, ground and cabin crew.
Steps ALWAYS available, not at a moment's notice but if the jetty was then U/s. Steps could have been put against the rear door.
Depending on many factors, the departure could be loaded in the same way, or the a/c towed to another jetty first. All in a day's work at an airport!