How does ConfigureWebHostDefaults work

In the following code snippet, I am having difficulty understanding where webBuilder came from. Assuming ConfigureWebHostDefaults is a method inside the parentheses, it should be a parameter being passed to the method. Within the parentheses, there is a function whose purpose I can understand. I also see that webBuilder is an IWebHostBuilder, but shouldn't we declare that variable somewhere before using it?

Here is the code snippet:

public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
              webBuilder.UseStartup<Startup>();
            });