This has some basic default .ycm_extra_conf.py files for various platforms and runtimes. If you have no clue what this is or why one would want to use this, go have a look at https://github.com/Valloric/YouCompleteMe and https://github.com/Valloric/ycmd .

Rename the Windows 10 1803 - VS 2017 Basic .ycm_extra_conf.py to just .ycm_extra_conf.py and copy it to where you want to use it. Like the root folder of a C++ project with MFC or any other Windows C/C++ based project. Please Note that you will have to add in additional paths for your project to make the completion engine completely happy.

Rename the macOS-10_13.ycm_extra_conf.py to .ycm_extra_conf.py and copy it to where you want to use it. NOTE: Make sure to adjust any Xcode paths and macOS SDK paths to suit your needs.

Use https://github.com/rdnetto/YCM-Generator to generate a .ycm_extra_conf.py for various build tools.