Load custom files from folders?

Can I load custom files that are in a folder structure in the same folder as the unity game is build?

For example to load custom textures/materials/meshes.

Ok I think assetdatabase is what I need to work with? Unity - Scripting API: AssetDatabase

Unity - Scripting API: Resources.Load ?

Where you can load from, will depend on your platform. On a desktop, you can save and load from anywhere you want. For example, you can use File.WriteAllBytes() and to write a file any where you as a user have permission to write. Other platforms are sandboxed, so where you can read/write is greatly restricted. For example on iOS, you’ll likely have to use Application.persistentDataPath for reading and writing data.

But reading between the lines, if what you want to do is store some data that is specific to your game and needs to be bundled with your game (and can be loaded as a whole into memory), take a look at the TextAsset class. You can drop any file (even non-text files) into the Assets folder as long as it has the .TXT extension. You can link it to a specific game object by dragging and dropping onto a public TextAsset variable. Or you can place the file in Assets/Resoruces and load the file dynamically using Resources.Load().