I’ll explain my theory step-by-step.
1.) A person dies.
2.) If this person is a nobody, two reapers will come. Nobody in my word means that this person is not known by many people. Even he dies, no one cares to take his corpse to the funeral.
3.) The reapers bring the corpse to a big trash can which connects the human world and the zombie world. The god of death has their face hid in the black hood. Their skins are always hid under black clothes.
4.) The zombie world is a place for those dead people who are unwanted by heaven, hell, or even human world. They really are nobodies
5.) Sometimes, humans throw trashes into this trash can, and these trashes are sent to the Zombie World.
6.) Therefore, zombies use these trashes to build or invent things.
7.) Zombies have nothing to do. They are just waiting for the day they are completely decomposed by the environment. They have no need to eat or do anything to survive.
8.) In the Zombie World, a person can go through the hole on the wall. This is the same hole that is used as an entrance. There are several guard reapers in front of the hole. They do not let anyone out. There is a big pile of trash which comes from the Human World.
9.) According to the history of Zombie World, one man tries to go back to the human world. He discovers a way to pass the reapers. Reapers are not so smart. If a person can dress like them, he can pass them………. That man is Madam Sunny.
10.) Zombat and Aranya tries to find black clothes from the trash pile together. Luckily, it does not take much time to find one.
11.) There is no additional punishment for those who already escaped from Zombie World as Reapers are not that smart.
Here is a very rough sketch of my idea.
Red is for Dead people and Zombies.
Purple is for reapers.
Yellow is for the tunnel.
Light Green is for trashes.
Note: I also have an alternate history for 9.). Madam Sunny finds another hole that leads to the same exit. (But reapers have no idea about this hole.)
The reason why I add reapers is because they can be a first obstacle for Zombat and Aranya. The two try to pass this obstacle together. I think it gives Aranya more roles in the story.