How to get access/refresh token and download file?

Can someone point me in the right direction ?.
I have C# .Net Web API application and i want to download files from Box. I would like to use Box SDK to do the following
1> Authenticate wih the box.
2> Get Access token & Refresh Token.
3> Use the access token to call API and download the files.
4> If Acess token or Refresh token expires it should get new tokens.

I have seen some examples on Stackoverflow doing it, but they are using HttpClient directly instead of using Box SDK. ( Box may be using HttpClient internally but thats not the issue here)

The example provided here  are also not complete

Question
1>Can someone please provide any C# samples on how to do this?
2>The documenation here says i have to create & use public key & private key however none of the examples i have seen using public/private key.