A Windows 10 USB flash drive installer can be manually made using the following steps. Here, the assumption is Windows will be installed to UEFI boot.
Use the Disk Utility to erase the flash drive. Use table below to determine the proper choices.
Option Value Name: Wininstall Format: ExFAT Scheme: Master Boot Record Mount the Windows ISO file and copy the contents to the flash drive.
Use the Boot Camp Assistant to download the Windows Support Software. This should be done on the Mac where you are going to install Windows.
Copy the Windows Support Software to the flash drive. If you do this correctly, then the
BootCampdirectory will appear in the root directory of the flash drive.
When the Boot Camp Assistant is used to install Windows, some of the installation files can be produced by the Boot Camp Assistant. These files aid in the installation of Windows. Without these files, Windows can still be installed, but some aspects may need to be more manually configured.