I'm developing an UWP app but I'm not sure about the architecture underlying it. If I check the nuget packages, I can find "Microsoft.NETCore.UniversalWindowsPlatform" and this package has .NETCore as its own dependency. So what is the exact relationship between UWP, .NETCore, .NETStandard and .NETFramework? And what is exactly the nuget package that I early mentioned? Thanks to all who will answer me.
You can use .NET Core to create an UWP-App but UWP itself isn't part of .NET. It is part of Windows 10 (Mobile) and its purpose is to develop universal apps that run on Windows 10. You can read this for additional information.
Therefore you can use .NET Core libraries for your UWP-project (as stated here) but you won't be able to use your application on another OS than Windows 10.
There were some rumors that Microsoft has plans on UWP for other operating systems but I can't confirm that. Probably outdated.
The nuget package Microsoft.NETCore.UniversalWindowsPlatform contains a bunch of packages and references that are needed in a UWP app. You can view them here.