In the Alchemy bake shop you can combine any three items of food from your inventory to create something new. The resulting item/food and it's number of stars will depend on the combined items. Once you have selected the items you want to fuse by putting the three items in each circle of the triangle, you will have a different percent chance of getting different items. There are hundreds of items in Pakka Pets, and thousands of potential combinations.
You can keep track of all the items you've discovered on the Item Book on the upper left corner. Just teach me how to make donuts