When did the first films for the universe containing the Arrowverse start production?
Arrowverse:  The shared universe, much like the DC Universe in comic books or the cinematic DC Extended Universe, was established by crossing over common plot elements, settings, cast, and characters. DC Extended Universe:  The films have been in production since 2011 and in that time Warner Bros. has distributed four films with more than ten in various stages of production.
2011