Examples of universe formation from two identitcally sized universes
Universes die eventually, be it through any method really, so obviously, new Universes need to replace them. Universes (like Hyperverses on the opposite end of the Archverses) have a surprisingly large amount of possible ways to get created. The most common method of Universe creation is natural, as there are plenty of Universes in a Multiverse and civilizations rarely run out of Universes. Even when artificial Universe creation is necessary, it is done in large bulk amounts at once in specific areas.
When two Universes collide with any speed that is reasonably above 0, the energy released is always enough to create a new Universe, but it doesn’t mean that one is bound to form. If a Universe is collapsing in on itself, it can create a sudden very large release of energy and matter, creating a sort of big bang, where everything previous is reshuffled, but the same laws apply. Universes attract and repulse each other with forces and there are many between them, so if a divergent imbalance happens, at some point the recipient of the forces breaks down and excess built up chaos can create multiple Universes, as a sort of grand burst or cosmic egg.
In a lot of cases, the beginning of a Universe is sparked by a big bang, as an incredibly tight and dense point causes itself to inflate, either a very short time after birth, or instantly. Universes that create a big bang upon their collision separate faster than they collided, due to the extra push generated by the event.
The Big Bang Theory, detailed and explained.
Universe births don’t usually happen when one Universe collides with another that is much more massive, as the smaller one bounces off rather easily. It only happens when one is approaching at speeds much greater than regular. If a smaller Universe is approaching a larger one at a metaphorical “bullet speed”, it can completely obliterate itself and, depending on how much larger the recipient is, the Universe it collided with, causing many Universes to be born, and if the larger one survives, it becomes one of very few Universes to have its own set of Universes inside, a sort of pseudo-multiverse if you will.