vcruntime contains the code that is linked into ucrtbase (in VS it is also provided as vcruntime140.dll)
vcstartup contains the code that is statically linked into executables that link to ucrtbase.dll. In Visual Studio this is part of msvcrt.lib (the import library for msvcrt), similar to our current msvcrtex, and it gets linked when you link to ucrtbase as well. The name is based on the folder name in the library.
Both libraries share some code, but each file is only compiled once.
