If you want custom variables in the .env file created on deploy, each WP site will create env variables using the wordpress_env_defaults from deploy.yml. To override or add to these env variables, you can edit group_vars/<environment>/wordpress_sites.yml.
For example, if for example.com you wanted to override the default db_name and add a new var new_env_var, you could add this for the site example.com:
wordpress_sites:
example.com:
...
env:
db_name: override_name
new_env_var: foo
(Note. If you’ve already created the db and then change the db_name, the deploy won’t be able to connect to the db unless you use the original name. To use a changed db_name you’ll probably need to rerun server.yml or maybe rebuild the server completely.)
You’ll find the created .env file on the remote at /srv/www/example.com/current/.env:
DB_NAME='override_name'
NEW_ENV_VAR='foo'
etc.
Edit. If you are creating or overriding sensitive env vars like passwords, use group_vars/<environment>/vault.yml to define the vars instead.
See also Where and how are values from .env defined as PHP constants