[How do the Yun reproduce?]

16 minutes ago, 鳳綺妃 said:

there's even indication that they may be from an entirely different realm

 So they are the eternal beings and as they wander through the time and space they happened to visit BnS realm. Unaware of death nor aware of life, they do not breed as there is no need to, they simply exist. Thus they continue their endless march throughout the universe, hoping for... well, who knows?