Disclaimer: I’m not an expert, I will edit my answer if I’m told I speak BS
My understanding is that SFOS on Sony controls the hardware with using closed binary drivers written for Android ( though an adaptation layer, libhybris), very much tight to a precise Linux Kernel version.
This is why there is a set of blobs from a specific version of AOSP (11 btw) that are flashed at the same time as Jolla’s SFOS ( see the installation instructions).
When flashing these (SFOS + ASOP bits) they will only erase some partitions in the phone storage - some others remain, and I suppose their content is needed somehow, esp. for the AOSP part flashed with SFOS. The content can be anything from config files to hw specific microcode.
These “relics” of the original Android install (11 or 12) may obviously cause behaviour differences - in my limited experience, I didn’t notice any. Given the closed nature of the AOSP blobs, I’d say it’s very hard to say what and how is used from the “relics”.
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