The API is not accessible from outside of your n8n.cloud instance but your instance can still access it by using localhost. So for example: http://localhost:5678/rest/workflows
And even if you want to access the internal API from outside your instance you can still build your own API proxy with n8n. That would be just a two-node workflow.
- Webhook Node: Entry point to your custom API, make sure to protect it with basic auth. Set up to return the data of the last node.
- HTTP Request Node: This node calls the internal n8n API and returns its data.
You can then access all the n8n.cloud instance data from anywhere.
Hope that is helpful.