You need one main file and one cores file.
If your OS is 32 bit, you get:
RetroArch.7z200 MB
Version 1.15.0-x86-weekly
or
RetroArch Windows x86272 MB
Version 1.15.0-x86
and
RetroArch_cores.7z173 MB
Version 1.15.0-cores-x64-weekly
If your OS is 64 bit, you get:
RetroArch Windows275 MB
Version 1.15.0-x64
and
RetroArch_cores.7z173 MB
Version 1.15.0-cores-x64-weekly
In case you are unsure which version your OS is, click the start button and choose Settings, then System, then About. One of the options listed under Device Specifications will be System Type. This will either be 32-bit (x86) or 64-bit (x64).