Sorry if I wasn't clear. I think a example is need for my point. Lets pick a saw, a basic tool for woodworking, only made with steel and wood. The wood part I think anyone could do it. But the steel you need to know you make a good enough steel, how to get the materials to process the raw materials and process the materials to make a saw. See each step it's more difficult to get, in at least first time, to another. In a middle a nowhere it's possible various factors like nobody know how to make a good coal to bake the steel, how see the temperature, etc.
And for the knowledge it's change over what the person/community need/wants, so basic knowledge maybe be forgotten over time or get less accessible. One good example there was a prototype of steam motor in Alexandria, but when it got in English hands trying to change the horse power it's started the the industrial revolution. Or if you want a good farm desert of times of your grandparents but nobody know how to do it and you got only a recipe that isn't an good explanation.