When module "Single Endpoint" is enabled, if module "GraphiQL for Single Endpoint" is enabled, then the single endpoint's GraphiQL client becomes publicly available.
To open it, click on link "🟢 GraphiQL (public client for the single endpoint)" on the plugin's menu:
By default, the client is exposed under
/graphiql/. This path can be modified on the Settings, under tab "GraphiQL for Single Endpoint":