I am just writing about what worked on my new Nexus 5X which has Android 7.1.1. As I have read many answers about how to set up a ringtone and none of them worked.
I installed a file manager app and then I transmitted the custom ringtone MP3 from the old phone to the Nexus 5X using Bluetooth which put it in the Downloads folder that I was able to browse in the file explorer app. So far so good.
I then tried copying the file to the Ringtones folder in the root directory (/storage/emulated/0) but couldn't select it in Ringtone settings so I had to try reading some messages to work out where I could try to put the file to get Android to detect it.
After looking at a few places I ended up putting it in the Android/media/com.google.android.talk/Ringtones folder. After that the Ringtone was able to be selected in sound settings.