When we apply "from scratch" to cooking it means combining ingredients to make a finished food item. To make a cake from scratch we combine flour, sugar, baking power, eggs, oil, and whatever flavoring you desire to make the batter instead of using a box of cake mix, which is kind of like a cake "kit". In cooking I have never understood "from scratch" to mean growing the wheat and milling the flour and growing the sugar cane and processing it into sugar and raising the hens that lay the eggs and so forth so "scratchbuilt" to me would mean using readily available materials to create a thing instead of a prearranged group of components.