Where is the Cities: Skylines II user data folder and where should the Prefab files be placed?

Cities: Skylines II user data folder is located at %LocalAppData%Low\Colossal Order\Cities Skylines II\. Within this directory, there are various subfolders for specific purposes.

Prefab files can be placed anywhere within the user data folder, and they will load automatically. However, if a Prefab file is placed in a folder or subfolder that begins with a . (dot/period), it will not be loaded. But the preferred folder is StreamingData~.

How to get a CID for the Prefabs?

Every custom content in Cities: Skylines II requires a CID (Component Identifier). Each item created in-game automatically saves with a CID file, however externally made items won't work properly without a CID file. To obtain the CID for the Prefabs, follow these steps:

1. Open the game Editor.
2. Select any object (like buildings, props, etc) from the Add Objects menu and plop it to the map.
3. Select the object and on the details panel, click on the Duplicate button. This will add a duplicated object to your mouse pointer.
4. Place the duplicated object on the map, and delete the original one from the map.
5. Look at the Workspace panel on the left, and expand the Objects heading if necessary to view the Objects List. There should only be one item.
6. Click the save icon beside the duplicated object (suffixed with (copy)). This will save this duplicated asset to the StreamingData~ folder.
7. On the StreamingData~ folder, you'll see a folder named with the object you just saved. On the folder, you'll see one .Prefab file and one .Prefab.cid file. Delete the .Prefab file and rename the .Prefab.cid file to include the name of your custom content. For example, if your downloaded prefab is StarQInc.Prefab, your CID file name should be StarQInc.Prefab.cid. Place them in the same folder, according to the criteria mentioned above, and the item will load automatically in game.

There are numerous other methods to acquire a CID, but ensure they are generated within the game. It's advised against using random GUID generators, as they could become incompatible in the future, even if they currently function.