Leftover food on plate at Restaurant?

I've always wondered what restaurants do with leftover food, especially meat pieces, left on customers' plates. Do they just throw it out, or is it ever repurposed in some way, like giving it to animals or composting? I know hygiene regulations in many places might prevent reuse, but it feels wasteful to discard it entirely. Do some restaurants have specific policies, or does it depend on the establishment? Would love to hear from restaurant owners, workers, or anyone with insights into this!