Modified versions of Android like LineageOS or Pixel Experience that offer new features or newer Android versions for older phones.
Flashing new firmware almost always wipes all photos, messages, and apps.
Usually installed via a custom recovery like TWRP or OrangeFox using a .ZIP file. Important Warnings
A standardized version of Android that can run on various devices supporting Project Treble. These often use .IMG formats. Where to Download Official Firmware
Since there is no single "ISO" to run, the installation method depends on your brand: