What do airlines do with planes at the end of a multi-stop route? The question's kind of broad and simple, so let me be a little more specific. Take Southwest Airlines flight WN3223. This is a multi-stop, trans-continental route that starts at LGA in the morning, flies to DAL, then PHX, LAX, OAK and finally SEA with expected arrival times usually around 10:00 PM. This route runs daily.

Now, according to Flight24, sometimes the same tail number is used for the flight on consecutive days, more often there's a rotation. So the question is, what does the airline normally do with the plane between 10:00 PM one day and 6:00 AM the next (when a plane, if not the plane, is expected to be back in the gate at LGA)? Does Southwest just deadhead it all the way back to LGA? Do freight companies subcontract with airlines like Southwest to move boxes overnight on otherwise deadhead flights? Does the plane stay in Sea-Tac and is used for another flight number (possibly the reverse route)? Does the crew deadhead to a maintenance hub? All of the above? 

EDIT: This question might be similar to the mentioned duplicate but I think it's more specific. I'm asking, more or less, how an aircraft used to operate a route that takes it across the country during daylight hours is typically "reset" so that it can fly that route again. 

The assumption, of course, is that the plane does fly that route again on some cyclic schedule, which may itself be wrong; a look at the tail numbers available for WN3223 shows that none of the aircraft used for that route in the past 4 days have flown it again in that same time period, despite the planes used for older flights having ample opportunity to make it back to LGA for another run. So, I think my answer, at least for Southwest's routing model, is "the plane spends the night at SEA, then is sent on a new route number the next day, which may have nothing in common with WN3223 nor get the plane back in a situation to run WN3223 again". 

Airlines running non-stop long-hauls probably have a more static schedule; an A380 might fly from Dubai to DFW, then turn around and fly right back, and that one route may be the only thing the plane is scheduled to do for weeks at a time.
Deadheading is rarely done because it is a non-revenue movement.

The most common use of deadheading (that is, flying the aircraft empty) is done for maintenance runs.

At the end of a scheduled route, the plane is parked at the last destination.  Sometimes, the next flight around uses the plane (if the route can be serviced by the aircraft).

Typically, airlines try to "park" their planes at their hub locations (where they have favorable landing and parking fees) rather than leave the aircraft at another airport.

Flight operations and planning make sure the aircrafts are maximized on revenue movements.

To give you an example - the shortest scheduled commercial flight for the A380 is Dubai - Kuwait (EK857); and the return flight Kuwait - Dubai (EK858).

The inbound aircraft on the flight yesterday (A6-EOH) was inbound from Perth to Dubai; and then flew to Kuwait and then back to Dubai. The next day (July 1st), the same aircraft is scheduled for SFO. (See the flight log at flightradar24).