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.