How does CancellationToken propagate cancellation requests through a chain of async method calls?

A CancellationTokenSource creates a CancellationToken that's passed down through every async method in the call chain; any method (or an awaited library call like HttpClient or Task.Delay) that receives the token can periodically check token.IsCancellationRequested or call token.ThrowIfCancellationRequested(), and calling source.Cancel() propagates the signal to every consumer holding that same token.

async Task ProcessAsync(CancellationToken token) {
  for (int i = 0; i < 1000000; i++) {
    token.ThrowIfCancellationRequested(); // cooperative cancellation check
    await DoWorkAsync(i, token); // token passed further down the chain
  }
}

var cts = new CancellationTokenSource();
cts.CancelAfter(TimeSpan.FromSeconds(5));
await ProcessAsync(cts.Token);