When you search for a "Mac OS X El Capitan ISO download," many third-party websites offer pre-compiled ISO files. Downloading from unverified third-party sources introduces significant security risks, including:

: Install El Capitan inside VirtualBox, VMware Fusion, or Parallels Desktop to test older apps.

Clicking the link will download a file named InstallMacOSX.dmg directly from Apple's servers.

Before deploying your ISO, ensure the destination hardware or virtual environment meets these minimum specifications: Minimum 2 GB of RAM (4 GB recommended). Storage: At least 8.8 GB of available storage space.

To install El Capitan on a real Mac, a bootable USB is the most reliable method. OS X El Capitan - App Store - Apple

Open your web browser and navigate to the official website.