I have long used JabRef for managing my bibliographies.  It stores data in the BibTeX format, which is simple and clear.  It's particularly good for people using LaTeX, but JabRef is also just a good bibliography manager that works in most contexts.

I wanted a program that launched more quickly, and that had some different features.  With the help of Claude.ai, I created a new bibliography manager, Sūcī.  It's free and Open Source. Installers are available for Linux and Windows, along with simple instructions for Mac.  It's still growing, but it is already usable for real work.  Help yourself.