Is up to you how you define the criteria for applying or not a specific configuration, for example you can use a system based on the location of the imported asset or detecting keywords in the asset name to do several things such as adding scripts to objects, adding colliders to objects, or changing settings to textures.
In order to guarantee the availability of the
AssetPostprocessor scripts you are going to implement, it is good practice to use a DLL to group them and use the DLL inside the
Editor folder in your target video game project. If you don't use a DLL and something in the project fails to compile, your assets are not going to be configured as you was planned.