Each building (with the exception of research buildings) has a shipping area. All resources that are ready to transport are stored here.

Bigger production buildings also have a storage area reserved for incoming goods e.g. the fries manufactory has a storage for incoming potatoes.

Warehouses have both, but the storage between them is shared.

There are several ways to increase the storage of a building depending on the type of building:

TIP: For production buildings that is further down the production chain, like a fries manufactory, you have the option to upgrade the storage for incoming goods (as described above), so it can store more potatoes, or you upgrade the production lines, so it can store more fries.