This verse simply repeats for emphasis what has already been said. Their capitals and their branches refers to the three pairs, each with a calyx or bud on the central stem, and each with two branches. Shall be of one piece with it means that these capitals and branches are to be formed from the same piece of gold. They are not to be formed separately and then attached to the lampstand. (See the similar comment on the cherubim at verse 19.)
The whole of it refers to the entire menorah, or lampstand. One piece of hammered work of pure gold is literally “hammered-work one of pure gold.” (These terms are discussed at verse 31.) Contemporary English Version has “The lampstand, including its branches and decorative flowers, must be made from a single piece of hammered pure gold.” One may also say “Hammer the whole lampstand out of pure gold. This includes its branches and flowers.”
Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .