Demon realm.
A place where demons inhabit and live their daily lives. It has six demon lords that keep the land safe and a demon king that rules them. Each demon lord governs its own land, but ultimately they must answer to the king's orders.
Human realm.
A place that humans inhabit and it consists of many different countries. Each country is different in their own way and has its own culture. Some are more similar to others, whereas others differ greatly.
Each realm is its own continent that is separated by a sea and connected by two narrow paths that can be passed by foot. There are three places where the human realm can access the demon realm. One is the Middle sea that sits in the middle of two continents in a crooked oval shape. The second path is an ancient bridge up north that was built by an ancient race long ago. It stands atop the Grand northern strait that connects the Middle sea to the Grand sea. The third is a narrow southern path that can be passed by foot that separates the Middle sea from the Grand sea.
To enter the demon realm by the Middle sea, humans have to pass through one of the demon king's armies. The demon lord of the sea governs all the coastal regions of the demon realm, even those that span to the Grand sea. The demon lord of the sea Orpantus commands all of the Middle sea and never grants entry to those who aren't permitted, namely humans.
If one tries to enter the demon realm by crossing the northern bridge called Nexum, the demon lord of fire will make sure that those who dare to try and pass without permission will meet a cruel end. The demon lord of fire Ignis governs most of the region around Nexum in the demon realm.
If one tries to get to the demon realm by walking the Semita's passage in the south, they will be met by the grasslands of illusions. The Semita's passage is covered in grasslands with forests ranging from a little woods to a grand rainforest. This area is governed by the demon lord of illusions Umbra and she governs all of the Semita's passage up to a certain village in the human realm called Lessin.
Out of all three of the places that humans can enter the demon realm, Semita has the most people who return home alive. But the scars that they bear in their mind often take a heavy toll on their bodies and sometimes make the people who tried to walk Semita without the permission of the owner sometimes become a completely different person. As if the soul of the person who went in there be taken and replaced by something else.
Out of the three options, Semita is the most feared place for humans to enter. The inhabitants of the Semita region that lord Umbra governs enjoy the vast lands that can be used for agriculture.
In reality, Semita is anything but narrow. People just see it as something much more narrow thanks to Lady Umbra's powers. The people who try to land on the shores of Semita often lose track of how long they travel thanks to Lady Umbra's powers.
In case you are wondering about the passage by the Grand sea, there are two issues. The southern path is impossible to try and navigate thanks to a huge field that makes ships lost or return to where they came from. Any sentient life can't sail the southern Grand sea and expect them to reach the shores of the other sides. This applies to ships on both sides.
The northern Grand sea suffers from winds that will destroy any ship that tries to sail too far from the coastline. The islands in those parts are governed by the twin demon lords of wind, Ven and Tus. Together they protect the northern Grand sea from ever being sailed by unwanted quests. The people of Ventus are protected by these winds that their governing demon lords make.
Any norther or souther the ships try to go in each respective direction of the Grand sea are met by icy waters that will take every passenger's breath away. Both north and south ice regions are ruled by their own separate king and queen. North has its ice king Elric and south has its ice queen Herta. Once upon a time, these two were lovers, but that is a story for another day.
And as for the last lesson of the world, the reason why each side can't go horizontally to the other realm is because the Grand sea between the two continents is ruled by an unknown monster no one is able to control. It is said that the grand sea between the two continents has some civilization hidden in it, but no one has returned to tell a tale.