Why was this done in Kikar ha'Yarden?
Radak: The earth there is good for making these Kelim. Malbim - the Yam and Mechonos were from regular copper; they were made in Yerushalayim. It would be very tedious to carry them so far!
Why does it say that the king made them? Above it said that Chiram made them!
Kli Yakar citing Mahari: He feared lest they mix with 1 the Nechoshes Memurat.
Kli Yakar: Shlomo was there, to show how dear Hashem is to him. He himself poured the molten copper to make the Kelim!
Malbim: Because the copper was so expensive and refined, Shlomo himself supervised and chose the place for it.
What is the meaning of "Yetzakam"?
Rashi: He melted [the copper] and made [the Kelim] in their forms.
What is "b'Ma'avei ha'Adamah"?
Rashi citing Targum Yonasan: B'Ovei Gargishta (in the thick earth). Radak - this is good earth for furnaces in which to make molten Kelim. However, such big Kelim cannot be made in furnaces! Rather, he poured the molten copper into molds in the earth.