What happens to the return values while using Google Benchmark?

Below is an example from the Google benchmark repository. The same can also be found on the quick-bench

1 static void BM_StringCopy(benchmark::State& state) {
2 std::string x = "hello";
3 for (auto _ : state)
4  std::string copy(x);
5 }
6 
7 BENCHMARK(BM_StringCopy);

I am trying to understand 2 things here.

1. Which function does copy in line 4 call? and How do I know this? The call does not look like the one from the C++ standard. Rather, looks like a free function.

2. What happens to the return values of the function being benchmarked? I have functions that return std::map. Should I ignore these return values?