At the end of https://www.arduino.cc/en/Guide/Libraries its states:
Please note: Arduino libraries are managed in three different places: inside the IDE installation folder, inside the core folder and in the libraries folder inside your sketchbook
In Arduino IDE; under File - Preferences - Settings there is a field called "Sketchbook location"
There is a folder within Sketchbook location folder called "libraries" which I believe is "the libraries folder inside your sketchbook."
On Ubuntu; how do I find the path of IDE installation folder and the core folder?