In the Absolute Shounen verse, there is an Alternate Parallel Universe where things such as Magic, Monsters, and other Fantasy things are far more common. 

Every God or Goddess sends People there After their deaths AKA Isekai-ing them as it's A Universe Created for such A Reason. The Very first Humans in this world being the very first Isekaied Adventurers.

There are ways to Travel between these worlds such as the Nekoya Resturuants door to this world which act as A Gateway between the two, Gates that act as A Passageway between the two worlds, and even Creatures such as Powerful Dragons or very Powerful Wizards can temporarily create Portals between their worlds but Usually it would drain them of their Magic keeping it open.

This World is where Tohru came from and it has characters in other series such as Konosuba, Tensei Slime, and even Resturunt to another world.