Apple no longer lists El Capitan in the main App Store, but you can still find the installer via Apple’s direct link (requires an Apple ID that previously “purchased” it).
The installer is officially distributed as a file rather than a standard ISO. For use in virtual machines (like VirtualBox or VMware), this DMG can be converted into an ISO. How to Download OS X El Capitan 10.11 (DMG, ISO, APP) mac os x el capitan 10.11 6 iso download 720p
The request for "mac os x el capitan 10.11 6 iso download 720p" is structurally flawed due to the "720p" modifier. There is no "720p" Apple no longer lists El Capitan in the
Apple still provides official channels to download El Capitan, which is the safest way to avoid malware or corrupted files from third-party sites. How to Download OS X El Capitan 10
If you've downloaded the El Capitan installer (approximately 6.2 GB) and want to create an ISO file, you can use tools like TransMac (on Mac) or UUByte (on Windows).