Finding the correct zip file depends entirely on your hardware and your goals (e.g., returning to factory settings or installing a custom ROM).

If you are looking for Android 6 system files (like OTA updates for older Nexus devices), these are often distributed as ZIP files for manual flashing.

Before downloading an Android 6 ZIP file, ask yourself: Why? Here are legitimate scenarios:

For devices that aren't officially supported by Google, developers often package Android 6 into "Custom ROM" zip files. XDA Forums XDA Developers Forum