ClearFog PRO /dev/loop is ext4

Good day everyone.

Just got ClearFog with built-in eMMC card and I made my custom version of OpenWRT for it. I would like to ask why my version has /dev/loop partiotion as ext4 and OpenWRT build is f2fs.
I didnt make any changes in default OpenWRT files - just added some custom files - and have switched from musl to glibc
Im asking, because sysupgrade doesnt work and i think, is because of ext4

Second question
OpenWRT u-boot doesnt see the eMMC until I insert uSD card to the slot. Should I use Marvell u-boot instead of the one from OpenWRT?

Im realy out of idea even after reading how image generator works, so very sorry for duplicate Topic also here to elite of OpenWRT gurus.
I would like to know, how I can make sysupgrade working
Continuing the discussion from ClearFog PRO /dev/loop is ext4: