Which version of .NET is used for UWP builds?

My team is trying to wrap our heads around what exactly is going on during the UWP build process in unity.

When unity builds to UWP using .NET scripting backend, which .NET version is being used for that build? When unity builds to UWP using IL2CPP scripting backend, which .NET version is being used for the IL generation? How is this different for Unity 2017.4 vs Unity 2018.2?

Knowing the answer to these questions tells us a lot about what API's are available to us and which ones aren't.

Unity Version: 2017.4.x

Scripting runtime: .NET 4.x Eq

Scripting backend: .NET

Api Compatibility: .NET 4.x

Platform: UWP Hololens

Publish Settings, Compilation override: Use .NET Core