How do I locate the Library Folder to delete 'Legacy Developer: Hp' extension?

Choose Go to Folder from the Finder's Go menu and provide /Library or ~/Library as the path; if it's not in one folder, check the other.

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Beware if a 3rd party System Extension is sneaked to Catalina or earlier, then it might be locked in the later upgraded sealed macOS, and the only way to get rid of it is a "clean" install.