For an analogy, if one of inputs of your business uses trash of others (e.g. for recycling), then the people throwing out this trash are effectively your suppliers and the ones you must consider in your supply chain, because no matter if you have a business relationship with them or not, they do matter in your supply chain, so you have to treat them (and risks related to them) the same as all other suppliers.

Your supply chain involves everything you use and rely upon, no matter if its provided by a business relationship, taken out of the ground, provided by the government, made in-house, or found in outer space.