If you've downloaded the El Capitan installer from the Mac App Store, you can create a bootable USB drive using:
Apple does not officially distribute its operating systems in standard .ISO formats. The tech ecosystem natively uses .DMG or .PKG files for distribution. Mac Os X 10.11 El Capitan Iso Download
Mac OS X 10.11 El Capitan ISO Download: Comprehensive Step-by-Step Installation Guide If you've downloaded the El Capitan installer from