OpenBSD nano r6s

Hello List,
The Friendlyelec NanoPi r6s is indicated (https://www.openbsd.org/arm64.html) as an example of hardware running OpenBSD.
I have downloaded the "install75.img" and the "miniroot76.img" and copied them on a sd card.

However, the NanoPi r6s does not boot on sd card but boot directly on the internal emmc (where the FriendlyWRT OS is installed).
I have downloaded the Ubuntu 22.04 from the FriendlyElec web site and write the image on an SD card
and the OS boots successfuly.

I have seen somewhere that the u-boot should be replaced for the unsuported arm Soc but all
the examples are for other arm soc and not for the soc (Rockchip RK3588) used in the nanoPi R6S.

Do I have missing some obvious instructions? One could notice than the structure of
the SD card to boot Ubuntu OS is very different from the structure of the SD card where
OpenBSD is installed.

Thank you for your help,

Best regards