Apple maintains a support article (HT211683) that links to older macOS installers. While many newer OS versions direct you to the Mac App Store, El Capitan often downloads directly as a disk image or an installer package depending on the redirect.

You must perform this extraction on a Mac that is compatible with El Capitan. Apple Support Community 3. Creating a Bootable USB (Optional)

Open Terminal and paste the exact line below:

Most users searching for a specifically want a bootable disk image to install the OS on a wiped drive. When you download the installer from Apple, it arrives as an application in your "Applications" folder. To use it effectively, you need to create a bootable USB drive.

Apple’s expired certificate. The installer was signed in 2015, and certificates expire.

md5 /path/to/InstallMacOSX.dmg

Your download was interrupted. Do not use torrents or random file-hosting sites (MediaFire, Mega, etc.). Redownload using GibMacOS. Verify the checksum using: