Macbook HD has a Library Folder and a System Folder with a Library subfolder. They don't appear to be exact duplicates, but should both exist?

Yes.

(141504)

And every user account has a Library folder too. The basic simplified idea is that each Library folder holds important information such as plug-ins, configuration files, and so on. The System folder should be considered Apple’s and shouldn’t be touched - ever - and that includes the Library inside which is Apple’s Library which is specifically for the operating system. When programs are installed sometimes files need to be placed so they can be used by other programs or by other users that may also have accounts on the computer. These are placed in the Library folder that’s on the same level as the System folder. The Library that’s in your home directory also contains the same kind of files as the other Libraries but they are files that affect just your account and no other.

Please remind me how to change the hidden Library Folder on my home directory and make it visible

Found it - thanks