Basically, Apple does not support running Windows 10 on Macs older than the 2012 models. This does mean you can not run Windows 10 on an older model, just that you are on your own to figure out how.
Windows is designed to use older drivers when newer ones can not be found. The drivers for Apple hardware is included in the "Boot Camp Support Software" (BCSS). If Apple did not supply Windows 10 BCSS for your Mac, you should use latest available BCSS that Apple documents is designed for your Mac.
One myth I put to test was: "Downloading the BCSS using the Boot Camp
Assistant will produce newer software than directly downloading from the internet". I have iMac (21.5" mid 2011) running OS X 10.11.3. I used the Boot Camp Assistant to download the BCSS for Windows 8.1 to a flash drive. I also directly downloaded the BCSS for Windows 7 from Boot Camp Support Software 5.1.5621. I then used Windiff.exe to compare both downloads. They were identical. I am not saying this is true for all Macs, it just happened to be true for mine.
To install the BCSS after Windows is installed, open the BootCamp folder and execute the
setup.exe application. If setup.exe will not execute, right click on the icon and select Trouble compatibility.