Verse 15 already said that he made the pillars!
Gra: Here it refers to putting on the pillars.
Malbim: Above the verse discussed how the designs were made in molds. Now it discusses making them.
What were the two rows?
Rashi citing Beraisa of 49 Midos, Midah 22: They were of copper pomegranates 1 . The pomegranates were inserted 2 in the chains (Divrei ha'Yamim II, 3:16).
What covered the crowns?
Rashi: The pomegranates covered the crowns. Gra - each pomegranate was thicker than the chain from which it hung. Therefore, (a) [a row of pomegranates going around the crown] covered the crown at the Beten (the thickest part), and (b) there was space between the chains holding a row of pomegranates. Therefore, there was a second row, in order that the chains will totally cover the [top half of the] crowns.
Radak, Malbim: [There were two rows of pomegranates around one of] the caps [that] covered the crowns.
Were the crowns on top of the pomegranates?!
Rashi: The verse must be rearranged. The pomegranates covered the crowns on top of the pillars. Each crown was like two bowls. The bottom bowl's interior faced up; it was called Goleh - "v'Gulas ha'Koseres" (verse 41). The top bowl was bent on the bottom one, and its interior faced down. It is called Sevachah - "veha'Sevachah Shetayim Lechasos Es Shtei Gulos ha'Kosaros" (ibid). It was wide where they stuck together. It inclines (towards the middle and gets thinner) and ascends, it inclines and descends. It is called "Beten" (verse 20).
Radak: Perhaps there were other crowns on the pomegranates.
Malbim: There were two rows of chains from which the pomegranates hung, from their heads.
