But are all of these bookable by themselves? I know there are other places in the world where an airline from outside the country or region offers connecting flights after longhaul service using the same plane, where the connecting flight is not bookable on its own, only in conjunction with that longhaul flight.

There used to be someone who offered a connecting flight LHR-BRU (London Heathrow - Brussels) in this manner, but I don't remember who offhand nor do I know whether it still exists.