The UpCloud datasource consumes information from UpCloud's metadata service. This metadata service serves information about the running server via HTTP over the address 169.254.169.254 available in every DHCP-configured interface. The metadata API endpoints are fully described in UpCloud API documentation at https://developers.upcloud.com/1.3/8-servers/#metadata-service.
When creating a server, user-data is provided by specifying it as user_data in the API or via the server creation tool in the control panel. User-data is immutable during server's lifetime and can be removed by deleting the server.