These are not official test images for the RK33xx platform.
After writing the image, you must correctly configure the DTB used in the file (extlinux/extlinux.conf).
Version 20191125 kernel 5.
Images rk3328 and r3399 are universal, when starting from USB media images are interchangeable (the only difference is used for the image u-boot and UART console output settings). Tar archives for updating are also universal.
Note that the new generic images now use a single download directory.