How to add other .cs files to build?

I know it may sounds very simple, but how to add other, not attached to game object, .cs files to build?
One of my .cs files is using some other .cs files that are not attached to any game object. This files are probably not included to builded game so after building my game some files are missing and errors are thrown. I have all .cs files in /Scenes folder.
I am beginner with unity. Thanks for any help.

if you are using it it will include it, no need to “add it to scene”. all the non-monobehaviour clasees are an example of this, if the issue is some classes getting lost can you try disabling “Managed stripping level” in the player settings?