There are two ways to create a connection between resources in the Altostra Cloud Designer:
Click the source connection point and then click a possible target resource.
Hover over any resource you want to connect from and you'll see arrows. Choose in which direction you want to add a new resource with a connection and hover your mouse over that arrow to see a list of possible resources to connect to. After clicking on one of the resources from the list, the resource itself and the connection between it will be automatically created.