Hi,
Please refer to the section <5 Programming Images> of ANDROID_USERS_GUIDE.pdf
To use the fastboot_imx_flashall script to download images to SD Card.
$ sudo ./imx-sdcard-partition.sh -f -D /dev/sdX # can be as imx8mm, imx8mn, imx8mp, imx8mq, imx8qm, imx8qxp, imx95.
Regards
Harvey