Each gear requires a specific product to process into other goods.
The Cheese Master, for example, can make Cheese (default), Mozzarella, Feta Cheese or Goat Cheese.
To fill the Cheese Master, click on it a few times and watch it get topped up with Milk. The gear needs 30 seconds to produce 1 wheel of Cheese.
You can collect the Cheese, which then automatically goes into your Barn (bottom menu, Barn icon).
Just like with animals, you’ll soon click enough times to collect and refill the gear.
To change the product type, click on the Product Selector (circular panel) located in the upper left or upper right part of your gear — depending on how it’s oriented — and select another good.