A Singleton service instance once created is reused for the entire application lifetime, whereas a Transient service instance is created every time a calling method or dependent class requests for it from the container.

A scoped service instance is created only once within a request scope and is reused for all calls within that request.

services.AddScoped<IMyClass, MyClass>();

A transient service instance is created each time a calling component method or dependent class requests for an instance from the container.

services.AddTransient<IMyClass, MyClass>();

A singleton service instance is created only once during entire application lifetime, common across all requests.

services.AddSingleton<IMyClass, MyClass>();

A class can be registered as a service by adding it to the IServiceCollection instance under the ConfigureServices() method in the Startup class.

A class MyClass can be registered as a service in the below ways:

services.AddT<MyClass>(new MyClass()); (or)
services.AddT<new MyClass());
services.AddT<IMyClass, MyClass>(); (or)
services.AddT<IMyClass>(new MyClass());

Where T can be Singleton, Transient or Scoped.