This will show you how to customize the logging. The default logging only writes to the console or to the debug window. This is quite good for the most cases, but maybe you need to log to a sink like a file or a database. Maybe you want to extend the logger with additional information.
This part is about a feature you can use to create background services to run tasks asynchronously inside your application. Actually I use this feature to regularly fetch data from a remote service in a small ASP.NET Core application.