Nook has both a production and a sandbox environment. The sandbox environment is available at https://demo.nook.io and can be accessed with a developer account. The same login credentials can be used for both the production and sandbox environments.
Apart from login credentials, the sandbox environment is entirely separate from the production environment. In terms of functionality, the sandbox and production environments are exactly the same, except that the sandbox environment only connects to banking institutions' sandboxes, so no actual payments can be made.
You can find credentials for various banking institutions' sandboxes here.
For convenience, KYC is not necessary to open a Nook wallet and wallets will automatically be topped up when you make payments.
The sandbox environment has its own API. To get access to the sandbox API, you will currently have to set up a test company in the sandbox environment and contact Nook to enable API keys for said company.
The Nook API serves both the production and sandbox environments. When a sandbox key is used, the sandbox API is called, when a production key is used, the production API is called.