The incoming connector can communicate directly with other systems using their API, preferably this would be a REST API made available by the system vendor. Incoming connectors can also communicate directly with a database hosted in the cloud, or local databases.
The incoming connector initiates the communication and reads from the source system, it then adds new insertions or updates from the data source to the records in Controller based on the business logic defined.
All communication with the incoming connector goes through the Web Application Firewall.