In case you want to provide your own ElasticSearch cluster (or use AWS OpenSearch, as an example) you can define in the inventory file exactly one host as a member of the external_elasticsearch group (nested inside the external group) as follows:
all:
children:
external_elasticsearch:
hosts:
whatever.eu-west-1.es.amazonaws.com:
elasticsearch_username: admin
elasticsearch_port: 9200
elasticsearch_protocol: http # or https with port 443
external:
children:
external_elasticsearch:
Every hosts under the external group is not directly managed by the Content Services playbook and is required in the inventory just for the sake of architecture description.