The behaviour is due to the lack of a special voice chip, which is only installed in the higher-end Galaxy series. In most A series such as the Galaxy A16 5G, "Hey Google" therefore cannot activate the device from standby.
Official Samsung documentation shows that voice wake functions are usually only available with Bixby if the device has this function. A-series models usually do not have this function.
Google help pages (Voice Match, Assistant settings) also describe how to activate "Hey Google", but only on devices that generally support this function.