Libraries are also "addons" just with reusable code for other addons. So just put them in your "AddOns" folder as well.
As long as they got their own <AddonName>.txt file (called manifest), which loads the fiel ingame to the addon manager, they will be found automatically.
Here is an explanation how to install addons and how to find your "documents" folder:
https://www.esoui.com/forums/faq.php..._howto_install
-> Normally it should be c:\users\your windows user name\documents.