Who should bless whom?
Radak #1: The Kohanim should tell Yisrael in their Brachah "Yevarechecha [Hashem...]" (Bamidbar 6:24).
Radak #2: Above (1-2), it says "Barechu Es Hashem." If you do so, He will bless you.
Why does it say "mi'Tziyon"?
Radak: The honor is there; the Brachah comes from there to all of Yisrael and to the entire land.
Malbim: The Brachah will not be via the Ma'arachah and orders of Shamayim, rather, from Tziyon - via the fine vapors and drops of dew that rise from Tziyon via their Avodah.
Why does it say "Oseh Shamayim va'Aretz"?
Radak: He makes Shamayim, which gives Brachah in the world, and He makes the land, which gives its produce for a Brachah, when He commands.
Refer to 121:2:2:1-2.
Malbim: This Brachah is not drawn from Shamayim to Aretz. Rather, it is from Hashem above. He makes both of them, and influences miraculous Brachah of Hashgachah. Malbim: (a) He makes nature, and what is above nature. Natural conduct and conduct via Shamayim, i.e. the Ma'arachah, are attributed to Oseh ha'Aretz. Wondrous conduct is attributed to Oseh ha'Shamayim, and He conducts it from a power above it. (b) The help looks like it is from the land (nature), but really, it is Heavenly, and of Hashgachah.
