Firmware Updates?

I am curious to know as to how/when/and by who, are firmware updates made to unsold (in stock) products.

Let’s say that a new problem solving firmware update has just been announced to owners of a new Canon HDSLR model camera.

You can go online and download and install the update.

Now what I would like to know is what happens to the very same Canon cameras that are still in stock and on the shelf at Amazon before the latest firmware was announced.

How does the new firmware fix get installed to existing stock?

Maybe Amazon continues to sell the unfixed cameras (as is) and the buyer has to learn about the new firmware by chance….and then download it?

Of course my questions goes to all product firmware updates.

Thank You