if Hikaru is made out of liquid metal, a-la T-1000, why does she eat, and how does she digest the food?
The answer, I think, is that she isn’t made of liquid metal. The alien technology is much more advanced than that, crossing into Clarke’s Law territory. A Ribers can assume multiple forms; when she is in human form she is indistinguishable from a flesh-and-blood human.
(Conversely, when they merge Tsubasa is no longer flesh and blood. That’s the main reason why she finds the process so frightening.)