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This is an update to the OS image, which adds a first-run script prompting you to create a new user.

Existing installations will not be affected.

Raspberry Pi OS does not have an "installer".

It has an graphical imager tool that presents options for configuration onto your SD card - I feel that it’s fair to call that an installer.

Its not though, something else is already named that, and it doesn't have one

Yeah. That's an installer.

An installer is a script/program which helps the user install the software, in this case the OS is the software.