hello tianhuxiaozi,
you may found the script file to generate the SD card flashable image for jetson-nano and also jetson-xavier-nx-devkit platforms.
for example,
$OUT/Linux_for_Tegra/tools/jetson-disk-image-creator.sh
please also check Write Image to the microSD Card for detail steps to prepare your microSD card with Etcher;
you may also check similar discussion thread, such as Topic 80283, and Topic 109010 for reference,
thanks