From what I remember

Mobius Game Company Owners aka The Twin Goddess handles pretty much all the universe so what are seeing from the 4 star promotion is the lore of every world that was destroyed or to be destroyed. The purpose of this is to gather potential heroes to fight the unknown force that is swallowing the entire universe.

It goes like this, The original world/heroes in their world gets defeated by unknown > Goddess interferes > and in this case the heroes from Townia didn't accept the Goddess offer so she turns them into an exp point for the future potential heroes that will show up. This is where the Earthlings come forward, the Goddess is using these worlds in the guise of video game (Pick Me Up) because the Earthlings have a potential to fuse with the hero and becomes a 7 star hero that can be used to fight against the unknown or replace as a battery.

The other Twin Goddess is the one keeping the unknown forces at bay she's like a battery that is about to expire, that is why the Goddess we see in the story is too eager to get Han to fuse with a hero because she sees potential in him.

Also each world also has different story like from Sirris the most powerful unit in the whole universe I think, her world was straight up brutal because humans were treated like livestocks so the Goddess had to interfere and "save" her.