On a vanilla server, a world consists of three so-called dimensions. These dimensions are the overworld, the Nether and The End.
The main world is saved in the base path of the world folder. The End is saved in the DIM1 folder in your world directory and the Nether is saved the DIM-1 folder of your world directory.
All Bukkit based server software (Bukkit, Spigot, PaperMC) uses a slightly different world format. As soon as you start a vanilla server with a Bukkit based software the "DIM1" and "DIM-1" folder are moved into separate folders for each Dimension: world_the_end and world_nether.
If you want to convert your world back to the vanilla format you need to move world_the_end and world_nether back to their original places.
On Aternos, this is done automatically for you as soon as you switch the server software. But if you want to download your world and play it in single-player mode, you have to do this manually:
- Download all worlds on the worlds page.
- Unzip all the worlds
- Open the main world folder and delete the DIM1 and DIM-1 folders if they already exist
- Move world_the_end/DIM1 and world_nether/DIM-1 into your main world directory
After that, you have a vanilla compatible world which contains the Nether and End and can be moved to the saves folder of your local Minecraft installation.
Please note that this only works if your current server software is a Bukkit based one.