Most manual firmware installations (except official emergency recovery) require you to unlock your bootloader first.
If you are looking to "download firmware" for your Android device, you are likely looking for one of the following:
instead, they typically use .ZIP files for recovery-mode flashes or .TAR / .BIN / .IMG files for tool-based flashing. While ISO files are standard for PC operating systems (like Windows or Linux), Android firmware is distributed as "ROMs" or "Factory Images" tailored to specific hardware. Understanding Android Firmware (ROMs)
Flashing new firmware almost always wipes all photos, messages, and apps.