• “In the beginning there was nothing, which exploded”. Into a chaotic multiverse. In at least one corner of the multiverse, an Egg spawns, somehow.
• Arceus comes out of the Egg. Arceus observes an universe of randomness, seeks to see the end of all things.
• Seeing the pieces of its egg as the only entities with order in the universe, Arceus codifies within them the rules to have energy and matter interact, thus creating from the pieces of the Egg the various energy types, and the pieces of the Egg become “Plates”.
• Arceus creates a “language” to mold the universe with, and six agents to process various facets of existence: these would eventually become the Creation Trio and the Lake Trio, known as a group as the Sextet.
• The “language” of creation eventually coalesces into its own concepts and symbols, and Arceus gives them shape and focus in the form of the Unown.
• Creation of dimensions, realms, galaxies, and worlds within them, for the Creation and Lake Trios to practice their powers in.
• Over time the Sextet maps the universe and marks various points of interest, one of them would likely be the Pokémon World.
• Arceus seeds various marked spots with the potential for life and, its energy for creation spent, goes into a deep slumber leaving to the Sextet the task of spawning, managing and evolving life and sentience.
• After millions of years, the Sextet finally produce the first prototype beings, essentially biological constructs with not much of a sentience. Because of their lack of experience, these creatures were very powerful but not autonomous or capable of self-preservation, and soon eradicated themselves and the worlds they had spawned in.
• A number of tries eventually lead to the creation of the first two fully biological, sapient and autonomous beings: the creation of the Lake Trio was a creature similar to them in appearance and capable of testing any further designs the Trio came up with, which they called the “Miju” (Mew); and the creation of the Creation Trio was a creature similar to them that embodied power and the ability to transform the landscape, which they called the “Drao” (Original Dragon).
• The rules of creation that were given form as the Unown, in their search of a purpose for the rules eventually spawned their own creation, a creature born from the ability itself to reshape reality; this creature would eventually come to be called the Feyt (“Original Fairy”).
• With the successful first biological creatures, the Creation and Lake trios bootstrapped life in more worlds and introduced more designs, though because of their lack of experience they were not really capable of checking up on their progress or of adequately caring for them.

Further detail.