I have a friend who works for one of the big 5 rental companies, and can somewhat summarize what he's told me a few years ago ( I do not know if this varies by company.)

All cars are 'owned' by individual branches/districts/regions, not the parent company. After a car from Location A goes one-way to Location B, when Location B rents that vehicle, they must share the revenue with Location A. (As mentioned, Location B will do everything they can to send that car back out on another one-way rental to avoid sharing revenue.) This is why in-town rentals you get are more likely to be locally registered and one-way rentals are more likely to have out-of-state plates.

After a set amount of time which I don't remember, Location B actually purchases the vehicle from Location A. I do not remember if this is a internal company purchase entirely on paper, or if they re-register, title, and insure.