Everything You make gets Your copyright by default. This means the default is: nobody can use what You make.
You can put other copyright terms on your works by adding a copyright notice.
You can put something in the Public Domain by adding a notice to your work.
Things can also become Public Domain automatically after a certain amount of time. But that time has been extended several times in the US to something unreasonable: like 75 years after the death of the author. You can thank Disney/Mickey Mouse for that.
This is not true:
No copyright notice == Public Domain.
Everything You make gets Your copyright by default. This means the default is: nobody can use what You make.
You can put other copyright terms on your works by adding a copyright notice.
You can put something in the Public Domain by adding a notice to your work.
Things can also become Public Domain automatically after a certain amount of time. But that time has been extended several times in the US to something unreasonable: like 75 years after the death of the author. You can thank Disney/Mickey Mouse for that.