OK to start out the term open bolt used in airsoft usually refers to a WE system that works exactly like a normal firearm's closed bolt. When the last BB is fired a the mag follower trips the bolt catch and holds the breech open. The older system called Closed Bolt was more like a gas powered AEG.
Open bolt SMGs in airsoft are less common. Below is a video of a Tommy Gun. Since most SMGs don't have a catch they will continue to fire when the BBs are gone but there's gas in the mag. Some airsoft SMGs will have a catch built into the mag that's not 100% accurate to the real thing but will stop the dry firing.
https://www.youtube.com/watch?v=IbVknGQapgo