If you’re on macOS, you can check what’s inside an image with Disk Utility. Open it, choose File -> Open Disk Image…
It means that if you’d like to have fat32 instead of fat, you have to provide another image with this type of partition. As I explained, balenaEtcher just copies the image as it is.
I’m sure that there are other tools for Linux / Windows platforms that will allow you to check what’s inside the disk image you’d like to flash.