The terms you're looking for are SVOD (Subscription Video On Demand) and TVOD (Transactional Video On Demand). Both business models are used by major streaming services. SVOD catalogs vary from month to month, depending on what the SVOD provider has licensed at the moment (although the heavyweights have their own content which is always available on their SVOD service). TVOD (purchase or rent a license to the title) is what Kaleidescape, Vudu, Apple iTunes, Amazon Video and others do, although this gets confusing because Apple also has an SVOD service (Apple TV+) and Amazon Prime subscribers get access to a catalog of "included with Prime" movies and TV shows. Now AVOD (Ad supported Video On Demand) is the talk of the town.