Thanks for those videos ! it shows what you've summarized to me thru PM or stm32duino forum.
So, your feet are wet now !
I'm waiting for my module ordered yesterday ... How long it took for you between order checkout and delivery ?
About the filesystem located in the flash, during the video, I saw that the header signature/structure doesn't look like SPIFFS, but maybe they use a custom version or similar tool, so don't expect "mkspiffs" to work as is with there firmware ...