When evaluating how your app works with its dependencies, remember::

The static cling code smell ( Static Cling | DevIQ)

When classes make calls to static methods or access static properties which have side effects or dependencies on infrastructure, static cling occurs.

• Example: A method that calls a static method that writes to a database. The caller is now tightly coupled to the database because anything that breaks the database call will break that method.
• Static calls that do not have any dependence on infrastructure (like calls that are completely stateless) are fine.

The aphorism “new is glue” ( New is Glue | Blog (ardalis.com))

• Each time a class is instantiated, consider whether it makes sense to hard-code that specific instance in that location, or if it would be better to request that instance as a dependency.
• Just as with static method calls, new instances of types that have no external dependencies typically do not tightly couple code.

Declaring Depedencies

In ASP.NET Core, methods and classes should declare their dependencies, requesting them as arguments. The Startup class is an example of this. Note: In ASP.NET 6+, the use of the Startup class is optional. Apps can be fully configured in Program.cs.

Configuring services in Startup.cs

The Startup class may have (but does not require) a constructor. If it has one, you can request dependencies through it. When the web host you have configured starts, it calls the Startup class:

public class Startup
{
    public Startup(IHostingEnvironment env)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(env.ContentRootPath)
            .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
            .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true);
    }
}

You can also request dependencies in the Configure method:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{

}

The ConfigureServices takes a single IServiceCollection parameter. It doesn’t support DI because its responsible for adding objects to the DI container and it has access to the currently configured services via the IServiceCollection.