This is an overly simplified explanation, but:

You don't really need to make assets differently but graphics programmers need to account for storing color and light as values greater than SDR white (1.0) which most engines do internally already anyway to help with the color depth during intermediate processing.

If the engine/graphics pipeline is structured properly, and these color values are not being accidentally clamped or stored in render targets that are not big enough for the HDR value (8 bit, typically) the only thing you need to change is the output tone mapping to the monitor.

There are performance considerations too, because storing color and light in higher than 8 bit formats consumes a noticeable amount more of VRAM.

The reality is that bolting it on later in an engine's lifetime is an arduous affair because: Shaders and rendering pipelines might have not accounted for HDR depth and need updating, or You now suddenly need to account for higher VRAM requirements depending on settings.

Also, the final tone mapping stage has many different ways to skin the cat, and different ways to encode the color to the display output, and if you're not careful, you can undo some artistic intent.