How come not all subjects of Ymir don't have royal (Fritz) blood? All of todays Eldians have a common ancestor, Ymir. But in all Eldians there is also a royal bloodline, Fritz/Reiss. This royal blood line came from King Fritz.  As far as we know, King Fritz and Ymir have three daughters. Thus should not all Eldians be also descendent of King Fritz? This only make sense if Ymir had kids with some-one else, other than King Fritz, but that is not shown. What am I missing? Or is it a plot hole?
To rephrase it.
We know Ymir only obey commands of the royal family and that why founding titan can only be used by those of the royal family. But if  all eldians are her descendants, and she only had children with royal King Fritz, all eldians should be Fritz. But that is not the case, why?
All Subjects of Ymir are Eldians, but not all Eldians are Subjects of Ymir or Royal.
Technically, every subject of Ymir has Royal blood, but it is not enough to make an impact. Remember, King Fritz was just a human and his bloodline can be diluted to the point where Ymir does not recognize it and all subjects are connected through Paths so that can not be diluted. Only one of the 3 daughters was kept as the Queen of Eldia and kept the Royal Family bloodline as pure as possible (possibly through incest, IDK. Historia was not a result of incest so it is a bit confusing). The other 2 were used as breeding machines to create more subjects of Ymir, diluting the Royal blood immensely.