They are located in the /etc/elasticsearch folder:
- elastic.co/guide/en/elasticsearch/reference/6.8/deb.html#deb-configuring
- elastic.co/guide/en/elasticsearch/reference/6.8/settings.html#config-files-location
There is also the etc/default/elasticsearch file: elastic.co/guide/en/elasticsearch/reference/6.8/deb.html#deb-configuring