It depends. Sometimes, on some short-hop commuter planes that I’ve ridden, those who are continuing stay aboard and new passengers (if any) board as soon as those who are leaving have gotten off. It’s a bang-bang quick operation.
On the other hand, I’ve ridden on international flights that do more or less the same thing, and we all get off the plane and (even though we technically avoid Immigration and do avoid Customs), passports are examined and we’re seated in an International Boarding lounge with nothing but the gate and an armed and guarded emergency exit, for up to an hour.
I would call the airline and ask them what to expect. (Of course, if you see from the itinerary that the scheduled landing at the way point is at 3:05 PM with a takeoff at 3:15 PM then you can assume that you all stay on the plane if you’re a through passenger.)
The plane can stop for many reasons: freight or mail delivery, to pick up passengers (whether or not they have shown up at the airport, the pilot in-flight won’t know), as a condition of the airport franchise and to maintain rights to the gate at the airport, for service or fueling of the plane (maybe even victualing) or for the crew to change. There can be lots of non-passenger related reasons to land at an airport.