How could a profile on classmates be hacked?

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Gmail is registered, there is a double authentication at the entrance.
I found a hack while looking through spam letters, well, I didn’t go there for three years and saw that I turned into Ksenia Petrova (the 13-character password was with different case and numbers-characters).
As I understand it, they cannot have anything else but brute force passwords, and they still have not bothered with protection. Someone got tired of mining, so I decided to use farms to brute-force passwords.