Building emmc image for sunxi cortex A7 / H3 (NanoPi Neo Core)

Hi all,
I'm trying to build an 22.03 image to be flashed into one NanoPi Neo Core.
There's ready images to be downloaded but only for sd card.
The board have an 8GB emmc that I would like to use.
The manufacturer provide an old emmc image to be flashed -> here:
I've followed the instructions provided ,to build a custom image that works fine but again is only to sd and not emmc.
Can someone please give me some hints / help how to do it? I've tried to find instructions with google but couldn't find any and being an old device I don't think the manufacturer will provide new images :frowning: