The version is tied to the Image Signal Processor (ISP) of the A-series chip. So the A17 has Smart HDR 4, while the A18 has Smart HDR 5.

Smart HDR uses neural image segmentation for tone mapping and other processing. In my opinion it goes way too far; trying to grab a faintly blue sky and make it as blue as possible, identifying a face and lightening any hint of a shadow, etc.

When people complain about iPhone photos looking over processed, this is why.