Core

Detect installed sdk

dotnet --list-sdks

Detect installed runtimes

dotnet --list-runtime

Switch core sdk in csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.1</TargetFramework>
  </PropertyGroup>
</Project>

Control version of net core used in a commandline, i.e. dotnet - global.json - searchable from current folder and above

{
  "sdk": {
    "version": "2.1.103"
  }
}

You can easily create a global.json file in the current working folder with the dotnet new globaljson command.
This will create a global.json file and pre-populate it with the latest version of the SDK available.
You can specify version that gets written --sdk-version argument

dotnet new globaljson --sdk-version 2.0.2

Seems no widely used helpers like nvm, only initial approaches https://github.com/faniereynders/dotnet-sdk-helpers