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Thanks. Somehow when I was learning English, the instruction was to use she/her to refer to an unknown third person, so did I when writing essays in a college here. That's why it felt strange when I saw they/their for 3rd person.

No, it was not standard conversational English to use the third person plural to refer to a known individual man or woman. It has been used to refer to a non-specific person.

Far from "standard", more like occasionally used.