it's fairly complicated but basically that's the magic of induced pluripotent stem cells. It's possible to turn human cells into cells that are prepared to become a specific organ, and then place them in an environment in which they will grow to that organ. This is because most of the necessary commands to differentiate and multiply cells into an organ are encoded in the genome and cellular state.

I think the wikipedia page helps understand this: https://en.wikipedia.org/wiki/Induced_pluripotent_stem_cell although the part where you can actually make a rat/human hybrid is still undefined science.