fhir-net-api is the GitHub repository for the official .NET API for FHIR. You can use this API in your .Net solution. It provides you with an FHIR model, FhirClient with methods for performing CRUD and searching, parsing / serializing methods, etc.
SMART at FHIR is a platform that provides you with FHIR profiles and a way to more easily integrate applications with clinical data, including authorization methods. They also provide an application gallery where you can host your own application.
Thus, you can develop applications / applications with both and a lot of work has already been done for you, but you still have to do the work of creating your own application logic.