From a legal perspective, the public domain is the space where no intellectual property rights exist. This means that works in the public domain may be used without any restrictions whatsoever.
This means that works can be used by anyone for any purpose without crediting the source or providing compensation. However, as a student, you are still bound by plagiarism rules, which say that you must credit all sources used. As a professional, you will be bound by ethical and moral standards.
In addition, the work has to be in the public domain in the country where the use will be available. If you publish a work online, and it uses a significant portion of another work, it must be in the public domain everywhere it would be available on the internet.
Works enter into the public domain in different ways. First, works whose copyrights have expired are in the public domain.
In Canada, this currently means 50 years after the death of the creator. It doesn't matter where the creator lived or created the work.
However, the rules are changing! As part of the recent USMCA agreement (which replaces NAFTA), Canada has 2.5 years (from July 2020) to implement a new public domain framework in which works are protected for life plus 70 years. This means that nothing will enter the public domain in Canada for 20 years after the term is extended. However, works that move into the public domain before this are not affected - they will remain outside of copyright.
Each country sets this period individually, so it is different around the world. The US has a much more complicated calculation that sets copyright for a much longer period. Copyright and public domain are determined by the country in which the item is being used. Wherever you intend to publish something, you need to abide by the copyright laws of that country.
Back to the Creative Commons blog for other ways that works can enter the public domain:
Second, works can enter the public domain if authors put them there before the copyright expires. The is made possible by using the CC0 Public Domain Dedication. This tool allows anyone to waive their copyright and place a work directly into the global public domain—prior to the expiration of copyright.
You don't have to apply a CCO license to release your works into the public domain, but it is an easy and clear way to do so. Once you relinquish copyright, you can't get it back. However, you may retain moral rights, which gives the creator certain rights despite not owning copyright anymore.
And finally one more way a work gets into the public domain:
Third, some works are in the public domain because they were never subject to copyright protection in the first place!
However, some of these might be trademarked.