Netflix nowadays produces a large number of original content quickly this is because they hire different production companies. Netflix does not make the content themselves they distribute the work among the production companies. In the same manner, Disney does not make Star Wars or the avengers they just finance and support Lucasfilms and Marvel studios. Each original show that you see on Netflix has its own studio that works for Netflix.
Netflix mainly acts as a financier and distributor of these original shows. In other words, they generate a project and finance it and in exchange, they get special distribution rights of the show, among other things that can be negotiated. The actual production or filming of all these projects can be done by other partners or production companies. Netflix does not essentially need to oversee the integrity of the actual production stage of a project, usually, someone else is hired to do that. Then when the projects are completed with production they go back to Netflix and present them with the final product, depending on creative control, certain parties will have more authority on final cuts and creative decisions. So, depending on that part, they can call for reshoots or develop the project more, or move ahead and line it up for release. They are reportedly targeting to produce an unprecedented 80 original films and about 700 original TV shows in 2018 alone.
This is the actual reason why Netflix now produces a huge number of content of original shows and movies quickly.
