Drupal 8 – what are best practices for installing external libraries?

On – Drupal 8 – what are best practices for installing external libraries?

In what subdirectory are people installing external libraries on Drupal 8?
– Note that practices for the location of contrib modules and so on have also apparently changed for D8.

How are module installers detecting external libraries and their locations, and configuring them on Drupal 8?

Drupal version: 


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