Hi,
Space is reserved at the beginning of the emmc for u-boot, and your kernel, initrd, etc... are located after this reserved space. When u-boot starts up, it does a bunch of initialization and then hands over control the kernel. As such, updating your kernel does not require a re-flash, as u-boot will find the new kernel by default. The document below is useful in understanding the storage topology.
http://opensource.rock-chips.com/wiki_Boot_option
Secondly, I should mention that there is a more discrete relationship between u-boot and the root filesystem, as it will read files in your /boot directory which can influence its behavior (boot.scr and armbianEnv.txt) or for some OS (extlinux.conf, etc), but you should not need to change settings unless you want to customize your boot process (change the boot logo, etc).
Lastly, some devices like the raspberry pi look for a dedicated FAT partition for the aforementioned boot files, but it looks like the Rockpi 4C looks for its root partition at 0x8000 as referenced in the document above. You can see the partition table to confirm by doing: fdisk /dev/mmcblk2 then pressing "p".