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

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.