The safest and most reliable way to get El Capitan is directly from Apple.
: Your Mac must be compatible (typically models from 2007–2015) and have at least 6GB of free space. ISO vs. BIN: Which is Better? The safest and most reliable way to get
: The native Mac format. Use this if you are creating a bootable USB for a real Mac. Creating an ISO from the El Capitan Installer How to download and install macOS - Apple Support BIN: Which is Better
: Best for Virtual Machines (VirtualBox/VMware) on Windows or Linux. It is a standard format that modern OSs can easily read. Creating an ISO from the El Capitan Installer
: A raw, sector-by-sector copy. While more "pure," it is rarely needed for macOS installers unless you are dealing with very specific archival software.
: Double-click the .dmg to find the .pkg installer. Running this package places the "Install OS X El Capitan" app into your Applications folder.