על צלעות הארון, on the sides of the Ark. The three times the word "the Ark" is mentioned in this paragraph corresponds to the three "Arks" i.e. the golden outer Ark, the one made of acacia wood, and the golden inner Ark, as we are taught in Yuma 72.

משני קצותיו, at the two ends thereof. Perhaps the verse wants to inform us that Betzalel did not make the lid of equal thickness throughout before pounding it with a hammer. He may have flattened it with hammer blows to correspond to the size required to fit as a lid over the Ark. He had added additional amounts of gold at either end so that he could fashion the cherubs out of that excess. This is why the Torah had to specify that the cherubs were made from the respective extremities of the lid.

ויעש את המנורה, He constructed the candlestick, etc. You will find that the Torah refers to the various furnishings of the Tabernacle by the same description both when we are told about what was required and when the execution is reported. Why, in this instance, did the Torah depart from this rule and instead of describing the candlestick as "the candlestick for illumination" or as "the golden candlestick," it is simply described as "the candlestick?"

Perhaps the answer may be found in the rule (Menachot 28) that under certain conditions the candlestick could be made from other metals, as hinted in the word תיעשה, (25,31). The Torah mentioned in chapter 25 that the candlestick had to be made of gold in order to remind us that the rule that it could be made of cheaper metals did not apply under the conditions prevailing then. It could also be that the meaning of the words "the golden candlestick" simply means that it was to be made of gold and not of any other metal although such a candlestick is acceptable in principle. In our context the Torah simply emphasised that Betzalel conformed to the instruction and made the candlestick out of gold as ordered.

ויעש את מזבח הקטורת, he constructed the altar for incense. The reason that the construction of the golden altar is mentioned here is because it was situated inside the Sanctuary as opposed to the copper altar mentioned afterwards. I have explained the reason why its construction had been commanded after G'd commanded the construction of the copper altar (compare 27,1 and 30,1 respectively) in my commentary on וכן תעשו.