How to create a commercial game using ready-made Unity templates without violating copyright? [closed]

This will depend on which license where the free templates are made available. If this information is not available with the templates, the only way to know it will be by asking the original authors. Unless the original author gives explicit permission, you should not assume that you can use a template just because it is free.

For example, a very common type of license to find for free assets is Creative Commons licenses. In less restrictive Creative Commons licenses the only thing the original authors ask of you is that you include their name in the credits of your game (CC BY). The most restrictive Creative Commons licenses may be that the authors of the models prohibit the use of the models for commercial purposes (CC BY-NC).