There are no parallel universes. There are other universes, and systems inaccessible from the universe you are part of. In some of your theories, you explore ideas of universes created by the splitting of decision paths. Sometimes, there is a splitting of a decision path, but no universe is created through this splitting. You can rather call it the creation of a different "line".

When you stand in front of a decision to go right or left in some rare cases you go in your perceivable "line" to the right and another line is created where you go to the "left". A second line of decision chains is created, not only followed by you but also by all other participants of it and all consequent decision chains. These splits can last very short (in your understanding milliseconds) or long (in your understanding years or decades). The short ones almost always collapse immediately and the results of all decision chains integrate into the original line. The longer ones might only collapse and integrate into the original line when you cease to exist on your line (when you die).

The creation of different decision lines is a systemic efficiency measure.