i think they are connected i reckon when a heartless and nobody are in contact they make a unbirth i should explain for example if roxas came into contact with sora when he was a shadow that they would merge and make a unbirth and this may happen with kh3 i was thinking of this and xemnas and xenhorts heatless may become a unbirth but this most likely isnt true and birth by sleep is where the unbirths are from so that will explain more about them
i think they are connected i reckon when a heartless and nobody are in contact they make a unbirth i should explain for example if roxas came into contact with sora when he was a shadow that they would merge and make a unbirth and this may happen with kh3 i was thinking of this and xemnas and xenhorts heatless may become a unbirth but this most likely isnt true and birth by sleep is where the unbirths are from so that will explain more about them