Here the first three times the word "he" is used as well as the word "him," the words refer to Festus. The fourth word "he" refers to Paul. The word "they" refers to the Jews who came from Jerusalem.
Here "judgment seat" refers to Festus' ruling as judge over Paul's trial. Alternate translation: "sat upon the seat where he acted as judge" or "he sat down as judge" (See: [[rc://en/ta/man/translate/figs-metonymy]])