The USB drive must be formatted with a FAT32 partition and the MBR partitioning scheme, after which you can simply mount the Windows 10 ISO in macOS and copy the files to the drive.
Formatting the USB drive can be done from the command-line fairly easily. First, run
diskutil list and find the identifier of the USB drive (this will be something like
disk3; make sure you find the right one, since you could erase the wrong drive and lose data if you don’t use the correct identifier). Next, the following command can be used to format the drive (replace
disk# with the actual identifier for your USB drive) and mount it as a volume named
diskutil eraseDisk MS-DOS "WINDOWS10USB" MBR <disk#>
cp -rp /Volumes/Windows10ISO/* /Volumes/WINDOWS10USB/
This command will take a while, and once it finishes, you can safely eject the drive and you should be able to boot from it to install Windows 10 on a PC.