Flash to mmcblk0p1 vs nvme0n1

I don’t understand how initrd related. not sure what it is.
can you explain if on 5.0.1 there is the option to do this:
$ sudo ./flash.sh --no-flash jetson-xavier-nx-devkit-emmc nvme0n1 ?
and, which of the notes on the ’ Boot from external drive’ is still relevant? how it should be updated for 5.0.1?

does flashing to nvme0n1 should not ‘populate’ to rootfs? because that the problem I see there is no new image file there when doing that