Are superman, flash,green arrow and super girl on the same universe?

DC believes in multiverse concept. That is there is more than one universe and more than one earth. DC believes there are infinite number of earth where different version of universe exists(To know about concept of multiverse click here). However according to DC we cannot see the other earth because they vibrate in different frequency than ours. DC superheros in TV all do not exists on the same universe. DC universe contains different types of crime fighting super heroes in different earth.

Who are on the same universe?

Arrow and flash have been known to be in the same universe(Known as Arrowverse). They are known to be participate in many cross over events. In fact they have helped each other in many crisis situations. Arrow helped flash to capture the reserve flash and flash helped the team arrow to escape from the league of assassins. Their tale continues so Flash and Arrow are on the same universe.
Superman and super girl exists on the different universe, this is specified when on season 1 of supergirl when flash accidentally runs to super girls universe and winds up helping her. While however they with the help of vibe flash often jumps between the universes and to help and get help. (Remember the time Super girl came to flash's universe to help them fight the aliens.)
To conclude Flash and Arrow are on same universe. Super girl and Superman exits on same universe but Flash, Arrow, Super girl and Superman do not exist on the same universe