System logs carried forward across upgrades?

iPhone 16, iOS 18.7.2 after upgrade from 18.7.2 to iOS 26.1

Are system logs carried forward across major iOS upgrades?

Context: probably beginning during a cable ISP area-wide outage of several hours duration, per cellular carrier, someone logged in to our cellular carrier account (before 2fa initiated, mea culpa) and over the course of about 4 hours, over most of which we were asleep, i.e. not using our phones, ~2GB of data were logged through the cellular network, putting us well above our monthly data cap. If possible, I want to determine the nature of the data that moved across the network from? or via? this phone, e.g. 1)Contacts exfiltrated? e.g 2)pinging associated with DDoS attack?. I thought a good start might be to review the system logs. Soon after we determined the spurious data usage was real, we updated from 18.7.2 to 26.1. I expect to go to a local Apple Store for assistance with this, but without logs from the time, the reason for the visit vanishes.

Thanks for your thoughts.