Didn’t know that when you run out of storage you can not boot your phone
TLDR; I have rebooted iPhone 7 with 100% full storage and now it does not boot. Be careful not to make the same mistake.
So my 256 GB iPhone 7 was almost full. Like literally full (there was about 400 MB left). Today I was leaving for holidays so before the trip I wanted to do local backup via iTunes (I do not use iCloud). iTunes gave me some backup error, I wanted to write it down on reminder or take a photo but it didnt let me because there was not enough space (100 MB, not sure where that additional 200 MB came during archieving), so I have decided to reboot my device since it usually frees some space (cache?). Unfortunatelly I got stuck in booting loop. Tried to hard reset and still in a loop. While on holiday I used my secondary Windows PC to download iTunes and attempted to update the software (or rather use the update option to fix it, since I was already using freshest 12.3.1), but I am getting unknown error 14 (tried different cable and different port). So now I am stuck in recovery mode, without any idea how to fix my iOS without losing data or how to recover my photos or even without knowing if I haven’t accidentally corrupt my backup (it’s on my home computer). Any ideas how to fix this sad situation? Do not make the same mistake.