Way back as far as 2004, a report to Congress on the rendering industry on what happens at the level of the independent rendering plantvis as follows:
“These plants (estimated by NRA at 165 in the United States and Canada) usually collect material from other sites using specially designed trucks. They pick up and process fat and bone trimmings, inedible meat scraps, blood, feathers, and dead animals from meat and poultry slaughterhouses and processors (usually smaller ones without their own rendering operations), farms, ranches, feedlots, animal shelters, restaurants, butchers, and markets. As a result, the majority of independents are likely to be handling “mixed species.” Almost all of the resulting ingredients are destined for nonhuman consumption (e.g., animal feeds, industrial products). The U.S. Food and Drug Administration (FDA) regulates animal feed ingredients, but its continuous presence in rendering plants, or in feed mills that buy rendered ingredients, is not a legal requirement.”
Remember that the fats (or tallow) are also used in plastics that leach into human food as well. Also, for your information these plants are regulated to be inspected by the FDA, but there is no law requiring when or how often. The drugs used to euthanize animals do not break down, so they get recycled into the feed and tallow as well.