Sentence examples for they mostly from inspiring English sources

"they mostly" is correct and commonly used in written English.
It is often used to indicate that a particular action or characteristic applies to the majority of a group of people or things. Example: In our office, they mostly start work at 9am but there are a few early risers who come in at 8am. In this sentence, "they mostly" indicates that the majority of people in the office start work at 9am, but not all of them do.