What is the ruling on using a phone that was found by chance, and from its messages, it was learned that it belonged to a girl who committed a shameful act?

The device you found is a lost item (Luqata) as long as you know it does not belong to the car owner. The rulings of Luqata apply to it, including publicizing it and announcing the find in places where people gather. Local newspapers serve this purpose. A lost item is a trust with the finder, who is not liable for it except in cases of transgression or negligence. If its owner comes and describes it, it should be handed over to them. If a year passes and the owner does not appear, the finder may donate it as charity or benefit from it by selling it or other means, whether they are rich or poor. If the owner appears after that, they either approve of the disposition, or they are entitled to its value from the finder.

The fact that the device's owner committed sins does not permit seizing her property, nor is it permissible to read the messages on the device, as this constitutes prohibited spying. Your action of taking the device and disposing of it immediately upon finding it is not permissible. You must repent to Allah and publicize the device while fully guaranteeing it to its owner.