It is implied that what they have heard is the great things that God has done in the past. Alternate translation: "As we have heard about the great things that God has done" (See: [[rc://en/ta/man/translate/figs-explicit]])
This means that they have seen the proof that the things that they have heard are true. Alternate translation: "so we have seen God do great things now" (See: [[rc://en/ta/man/translate/figs-explicit]])
Both of these phrases refer to Jerusalem. Alternate translation: "in the city of our God, Yahweh of hosts" (See: [[rc://en/ta/man/translate/figs-parallelism]])