What is "bi'Dvar Hashem"?
Radak: This is like "va'Yomer Hashem" in creation. It is Hashem's desire.
What is the difference between "bi'Dvar Hashem" and "Ru'ach Piv"?
Radak: It is a repetition.
Malbim: Ru'ach Piv ('wind' that leaves His mouth when He speaks) is lighter than Dibur. "Kol Tzeva'am" are the stars and luminaries that were suspended on day four.
Was the land not made via Devar Hashem?!
Radak: The land is included in "Shamayim" and "Kol Tzeva'am", for the sphere [on which the celestial bodies revolve] turns only on something stationary, i.e. the land 1 . Tzeva ha'Shamayim conduct everything on the land, according to Hashem's desire.
Malbim: The land is different. Shamayim did not change; it is like it was created on day one, including the luminaries, just they were suspended on day four. The land changed after its creation via the nine [subsequent] sayings of creation - the water gathered, vapors rose, and beings were created on it. "Hashem Borei ha'Shamayim Hu ha'Elokim Yotzer ha'Aretz v'Osah Hu Chonenah" (Yeshayah 45:18) - He added fixtures via changing forms. Regarding Shamayim it says "Yashar Devar Hashem" (verse 4), for nothing changed.
