If copyright expires all works enter to the public domain.

Four ways for creative work to enter the public domain

The copyright expires. According to the Bern convention the term of an individual’s copyright expires 50 years after the individual’s death, but for some countries this term can be longer, you have to be sure. Not all countries have signed the treaty.
The work was never entitled to copyright protection. For example, ideas and facts. Some additional categories according to the Berne Convention can be official texts of a legislative, administrative, and legal nature. Note that it is up to the member countries to decide if they exempt those texts from copyright protection.
The creator dedicates the work to the public domain before copyright has expired. A creator can decide to forego the protections of copyright and dedicate their work to the public domain. As described earlier.
The copyright holder failed to comply with formalities to acquire or maintain their copyright. In most countries the creator will automatically have a copyright protection on their work, and you don’t need to renew it. But this wasn’t always how it worked, “and many works have entered the public domain [..] because a creator failed to adhere to formalities”.

Video: What is public domain?

Public domain

Creative work where copyright no longer applies goes under public domain

The public domain symbol is not a license, but gives a visible signal that the work is free of all copyright restrictions, i.e. free to copy, adapt and share.

Public domain is symbolized by a crossed over C (copyright) in a circle

N.B.! There are some moral rights which may continue to exist after copyright expires.

CC Zero (CC0)

Public domain

Creative Commons has a legal tool called CC0 (“CC Zero”) Public Domain Dedication, so if you want to share your own creative work freely, you can apply the CC0 dedication to your work.

CC0, even if it is not a license, it also uses the three-layer design—legal code, deed, and metadata.

CC0 is symbolized by a zero within a circle.