These are commands. By commanding that the expanse should exist and that it divide the waters, God made it exist and divide the waters. (See: [[rc://en/ta/man/translate/figs-imperative]])
"It happened like that" or "That is what happened." What God commanded happened just as he said it should. This phrase appears throughout the chapter and has the same meaning wherever it appears.
This refers to the whole day. The writer speaks of the whole day as if it were these two parts. In the Jewish culture, a day begins when the sun sets. See how you translated this in [Genesis 1:5](./03.md). (See: [[rc://en/ta/man/translate/figs-merism]])
This refers to the second day that the universe existed. See how you translated "the first day" in [Genesis 1:5](./03.md) and decide if you should translate this the same way.