To build a Bridge, select it in a shop and then place on a free slot of the first island (from which the resources will be streaming). Only 8 island slots on its sides are available for bridge building. Approve the purchase.
Then the button with the red exclamation mark (!) will appear, meaning that the Bridge is not connected. Press this button to select the Bridge. Then drag the connection button to another available slot on another island.
If no possible connecting slots are available, it's possible to sell the bridge and start over in another place.
It's possible to upgrade the bridge to increase the quantity of resources transported.