The single most important recent change is the , introduced with VS2015. Before UCRT, each Visual Studio version shipped its own msvcrXXX.dll , leading to “DLL hell” — applications needing multiple versions installed. With UCRT, the standard C library functions became part of the Windows OS (starting with Windows 10), and updates come via Windows Update.
Microsoft CRT implements nearly all of ISO C (C89/C99/C11 except some C99 features like <tgmath.h> or complex math fully, but modern versions are catching up). Major families: microsoft c runtime