How does tx2 start up the init script(and where is the script) after loaded the kernel image?

I have been trying using overlayroot filesystem to protect the root file system. As now the R28.2 don’t use the initramfs, the simplest way to achieve the goal now is not go through . I have to try to modify the init script to mount the overlayroot on / by adding some cmd line in the init script like OpenWrt system?

So, can anyone show me some info?

FYI, this is being tracked in this thread (knowing how to get an initrd working is basically the same question as how to hook init in):