当我们创建好一个新的ASP.NET Core Web应用时，系统会使用CreateDefaultBuilder方法，这个方法完成了以下操作：
- use Kestrel as the web server and configure it using the application's configuration providers
- set the ContentRootPath to the result of GetCurrentDirectory()
- load IConfiguration from 'appsettings.json' and 'appsettings.[EnvironmentName].json'
- load IConfiguration from User Secrets when EnvironmentName is 'Development' using the entry assembly,
- load IConfiguration from environment variables
- configure the ILoggerFactory to log to the console and debug output
- and enable IIS integration
如果使用了CreateDefaultBuilder的有参数版本，则还会通过命令行参数来初始化IConfiguration。
以上内容来源于官方文档：microsoft.aspnetcore.webhost.createdefaultbuilder