Libraries directory is relative to which root directory?

Problem/Motivation

The root directory of "/libraries" is not clear in bootstrap_barrio_subtheme.libraries.yml

It's not clear if the "libraries" directory is found within the theme's root directory:

example.com/web/themes/custom/my_bootstrap_barrio_subtheme/libraries

or within the 'web' directory

example.com/web/libraries

File: my_boostrap_barrio_subtheme.libraries.yml

bootstrap:
  js:
    /libraries/popper/popper.min.js: {}
    /libraries/bootstrap/dist/js/bootstrap.min.js: {}

Steps to reproduce

Read *.libraries.yml
Check where the current libraries directory is.
Run code to see if bootstrap library is found.

Proposed resolution

Clarify intended location of the library directory.

Remaining tasks

User interface changes

API changes

Data model changes