Configuration shipped by distribution is in /usr/share/lightdm/lightdm.conf.d and local changes are in /etc/lightdm/lightdm.conf(.d) and it is this way in any other distribution using lightdm.
Because it is rather unclear what you are asking. You built some picture in your head but we are not aware of it and do not know what you mean with your questions. Anyway, /etc/lightdm/lightdm.conf is entirely optional and need not exist. I do not know why you are so fixed on it.
And once more - if you have questions how to configure lightdm you really should start new topic with questions how to configure lightdm.