[Xeamek]

core (as in kernel) is running linux kernel which is C, yes.

But java is not 'just UI' - majority of user space apps run java (or jvm, to be precise), as well as tons of system services, to the point where you can't actually do a lot of android API calls from native code, because a lot of logic is written in java(or kotlin) code, and not just java being wrapper around native code.