Beef liver supplements are freeze dried or desiccated organ meat pressed into capsules. They contain iron, but iron is one nutrient among many, alongside vitamin A, B12, copper, and folate.
Iron supplements, in contrast, are single ingredient products built around one iron compound, such as ferrous sulfate, ferrous gluconate, or ferrous bisglycinate. The core distinction is concentration and purpose. Beef liver gives you a food matrix. An iron supplement gives you a targeted dose.