Where can I find the Drush aliases ?

You can get all the details about your Drush installation by running:

* Drush 8

$drush status  --fields=drush-conf,drush-script,drush-version,drush-temp, drush-alias-files
 Drush configuration    :  /etc/drush/drushrc.php
 Drush script           :  /usr/local/drush8/vendor/drush/drush/drush.php
 Drush version          :  8.4.2
 Drush temp directory   :  /mnt/tmp/xxxx
 Drush alias files      :  /mnt/drush/xxxxx/xxxxx.aliases.drushrc.php

* Drush 9

drush status --fields=drush-conf,drush-script,drush-version,drush-temp,drush-alias-files
 Drush configs : /etc/drush/drush.yml
                 /mnt/www/html/xxxxx/docroot/vendor/drush/drush/drush.yml
 Drush script  : /mnt/www/html/xxxxx/docroot/vendor/drush/drush/drush
 Drush version : 9.7.2
 Drush temp    : /mnt/tmp/xxxxx
 Drush aliases : /mnt/drush/xxxxx/xxxxx.site.yml

When you want to override the Drush alias with a new file , you can easily find the path for the new file.