Is Universal Windows Platform the replacement of WinRT of Windows 8 and Windows Phone apps? I mean, there was a WinRT platform to develop metro apps exclusively for Windows 8. Now, that is replaced by UWP, isn't it?
That is correct, UWP is the new platform for ALL Windows devices going forward (Win 10+). However WinRT is not replaced by UWP but is instead an extension on top of it, making UWP a much broader set of APIs that can be used across even more devices. As Microsoft themselves state:
With this evolution, apps that target the UWP can call not only the WinRT APIs that are common to all devices, but also APIs (including Win32 and .NET APIs) that are specific to the device family the app is running on.
The UWP platform supports the "Universal Device Family" class of APIs which is then supported on ALL windows platforms (Xbox, Phone, Desktop etc). There are some extension families that you can use that will limit the apps reach, such as a "Mobile Device Family".
These specific device family APIs can however be checked for and used at runtime gracefully. For example you could show your own position using GPS on a phone, but not enable that functionality on a Xbox.
I hope this answer helps you, if you have any more questions about this I recommend reading this article about the UWP platform:
Source: https://msdn.microsoft.com/library/windows/apps/dn894631.aspx
Have a wonderful day!