No idea for the I Phone ...but my Samsung A36 , it is a new ish one so may be different to yours .
settings,
connections ,
data usage,
mobile data usage ,
Scroll down and it shows how each app has used it's data .
May help to narrow it down . But don't expect much from the service provider ...
I seem to remember you saying there was a lot of " stuff " on your I phone when you got it
It may have been compromised with malware and working in the backround .
If buying a second hand phone ALWAYS do a hard reset before setting up ...
But really , the price of cheap new phones these days that are perfectly capable , why ?
Yes you can get a better phone for the same money going used but a used one will run out of security updates sooner. I personally would not use a phone that is used for anything sensitive ( ie anything shopping / banking related ) that hasn't got valid security updates. Top of the range , from both , will be typically 7 years , lower spec 3/4 years of updates from original release...NOT when you got it
...... Anything older , and not able to be updated , will be more open to being hacked ........
Tinfoil hat back in the drawer ......
