The planes are scheduled sectors for the day, and the crew(s) pick the plane up according to their schedule. For example, in December, I flew EWR-TPA on CO on a 738. After the flight, I spoke with the FO for a while (the whole 30 minute turn) in the cockpit. I asked him about commonality of the different 737's, and he said they are qualified to fly them all, and that on this particular day, he had flown the 735, 739, and the 738. I asked him how this works, and he said that each aircraft was assigned where it would go each day, then the crews met up with the plane according to the sked. For example, he flew the 735 into EWR, picked up the 738 for EWR-TPA and for TPA-IAH.
As much as their sked that day dictates. The FO I mentioned above had switched aircraft 3 times that day. Some may fly the same aircraft for 5 sectors in one day, and some may switch 5 times.
If you don't stand behind our troops, feel free to stand in front of them.