In verses 18-19 the author tells the reader background information about how Judas died and what people called the field where he died. This is not part of Peter's speech. (See: [[rc://en/ta/man/translate/writing-background]])
Although Peter is addressing the entire group of people, here the word "us" refers only to the apostles. (See: [[rc://en/ta/man/translate/figs-exclusive]])