The word "them" refers to Cornelius and his Gentile guests and household. Peter does not call them Gentiles in his account to the Jewish believers at Jerusalem.
The word "they" refers to the Jewish believers to whom Peter spoke. The word "us" includes all of the Jewish believers. (See: [[rc://en/ta/man/translate/figs-inclusive]])
Peter uses this question to emphasize that he was only obeying God. AT: "Since God gave to them ... I decided that I could not oppose God!" (See: [[rc://en/ta/man/translate/figs-rquestion]])