The installation guides are always updated for the latest version of the build we share. We do not have installation guides for different OS version.
Flash manually means the build has to be downloaded from the link given in the installation guide and installed following the steps given in the guide.
The A16 build will not show up in the device installer as the updater app blocks OS upgrade builds.
The alternate is for official builds which get OTA OS upgraded. For example a device if on the official A14 build will be upgraded by downloading the build OTA (over the air). No need to manually download on a PC and then install following the guide.
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