I have one friend who is an astrophysicist and another I knew who passed away a bit ago. This is very short (and the details on the theory are VERY long) but it's an overview

Cosmic inflation: The theory of cosmic inflation, which describes the universe's rapid expansion right after the Big Bang, predicts a "multiverse" of bubble universes that are constantly forming.
This process could be eternal, creating an infinite number of universes.

Each bubble universe could have different physical laws and constants.

These universes would be separated by ever-inflating space, making them undetectable.

Fine-tuning problem: The fundamental constants of our universe—like the strength of gravity or the electromagnetic force—seem to be precisely calibrated for life to exist.

If these constants were even slightly different, complex structures like stars and galaxies would not have formed.

The multiverse offers a solution: if there are countless universes with varying constants, it is not surprising that at least one would have the "right" conditions for life, and we would naturally find ourselves in that one.

Many-worlds interpretation of quantum mechanics: This interpretation of quantum physics suggests that every time a quantum event with multiple possible outcomes occurs, the universe splits into separate universes, with each outcome taking place in a different one.

For example, if a particle can be in two states at once, the universe would split so that one universe contains the particle in one state and another universe contains it in the other.