File structure of libraries folder and detection of module external plugins The module CKeditor anchor link require a CKEditor plugin.
Can this plugin be placed in subfolder of the /libraries folder or does it have imperatively to be placed at the root of /libraries ?
More generally, does drupal parse recursively the libraries folder when intenting to detect external plugins ?
The libraries folder is no longer managed by Drupal. The project Libraries doing this jobs in older Drupal versions seems to be deprecated in favor of a composer based concept. Modules still using the libraries folder use their own logic and you have to follow their instructions on the project page or README.