How is a being created?
A being is created when someone like you or me puts their attention on something on a continuing basis. Eventually, this "attention" becomes a separate being, with an initial mission decided by its "creator".
As the being goes down the dwindling spiral, it becomes a BT, or as I like to say, a lambdan.
During the OT III incident, many thetans were pushed down into BT status by the cruelty of it all. Both Incident I and Incident II, in constant restimulation, kept them being BTs.
When you solo audit the incidents and free BTs, they may continue to be BTs (probably) and may or may not infest a different person. Or they may rise back up into being thetans, perhaps with a new goal of their own choosing.
The final act of any being is to decide its mission is over and merge back with the being that created it (I don't mean god).
The above is rather simplified but it's a start.
A being is created when someone puts the label "a being" on something.