UCRT: which contains compiler independent components (such as the standard c library, POSIX extensions etc))
vcruntime for compiler-specific functionality (code for program startup, exception handling, intrinsics)
ucrtbase.dll contains the UCRT and ships as operating system component with and since Windows 10.
The source code for the UCRT is installed along with Visual Studio, by default under C:\Program Files\Windows Kits\10\Include\sdk-version\ucrt. The sdk-version corresponds to a version (for example 10.0.1.14393.0 for the Anniversary update).
The corresponding (architecture dependent) *.lib are under C:\Program Files\Windows Kits\10\bin\architecture\ucrt (with, for example, x64 for architecture).
The redistributable (that is: non-debug) DLLs itself are under C:\Program Files\Windows Kits\10\Redist\ucrt\DLLs\architecture.