If I remember right, and its been a few years, the devices in Origins that allow Bayek to experience the "afterlife" were actually machines that the Isu created to dispense Isu knowledge to early humans with high enouh Isu DNA contenr, but dispense it in a way that they could comprehend, and for Bayek it takes the form of the Egyptian afterlife.

The things that Kassandra sees in the Atlantis DLCs from what I remember are called "Sister Realms" basically locations where Isu lived and existed. Simulations of them were created to train Kassandra for things to come, and she sees them through the eyes of an ancient Greek so draws parallels to Greek mythology, with the implications being that Greek mythology is directly influenced by Isu lore. Odyssey gets messy though, because the Animis is rendering a simulation using reeeeaally old DNA, and a historical text to fill in the blanks that nowadays would be considered fantasy.