vmlinux

IT is the Linux monolithic kernel in ELF format. This is binary and contains no unresolved references.

/arch/arm/kernel/head.o is the an architecture specific (ARM in this case) that perform low level kernel intialization. This is executed first when the kernel is loaed and passed control to by the bootloader.

init_task.o set up the initial thread and task structures that the kernel requires.

The largest object module making up kernels are filesystem code, network code, built-in drivers code and the kernel (which contains scheduler, process and thread management, timer and other core functions).

The /arch/ARM/kernel contains specific architecture functionalities such as low-level context switching, hardware level interrupt, processor exception handling etc.