They did go back to Jerusalem to wait for what we have in the next chapter which we call Pentecost.
Meanwhile there is work to be done. Judas (who went to his own place, verse 25) needed to be replaced as an apostle. There are disagreements about this citing Paul as the twelfth apostle. However these were for Jewish Apostles. Paul was for the Gentiles. These stayed in Jerusalem and Paul went out to the world. Matthias was chosen by the rest because he followed Jesus all the way through His ministry. There were two (Justus also), but Matthias was chosen so he became the twelfth apostle.