A few useful links concerning public domain music...

Here is a great page of easy to understand, general guidelines and information about public domain vs. copyrighted music from Wiki:

https://en.wikipedia.org/wiki/Public_domain_music

Another very good source of quick general info:

https://www.pdinfo.com/

And here is their very cool searchable list:

https://www.pdinfo.com/public-domain-music-list.php

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A couple other search sites that can point you to public domain/use-able/free music:

http://www.freemusicpublicdomain.com/

https://www.adweek.com/digital/royalty-free-music/