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?