How to modify initrd in rockpi n10 kernel

Hi ,
I what to choise rootfs after kernel bootup, so I want to make some change in initrd’s init program.
Can anyone tell me where’s the init’s source code in radxa’s kernel source tree? And where’s the source file for initrd image?

I find the init in /usr/share/initramfs-tools/init will be replaced into initrd.
But the new problem is how to output any log from the init script?