Why did the Torah have to tell us at this point in the Torah that Aharon set up the lights/ wicks on the menorah? This command seems to repeat Shemot 25:37, and Shemot 40:25 records that Moshe fulfilled this command of how to place the lights/ wicks in the menorah. Shemot 25:30 records that bread was to be placed on the table in the mishkan/ ohel moed, and the only time the Torah records that this instruction was fulfilled was also by Moshe in Shemot 40:23.
Why is there is a specific commandment to Aharon how to set up the lights/ wicks on the menorah and why does the Torah record that he fulfilled this command? The Torah does not record that Aharon brought the incense offering every day. Also, it is surprising that this information concerning Aharon’s role by the menorah is not recorded in the book of Vayikra which records the work of the priests. Various answers have been proposed to these questions, see Rashi, Ibn Ezra, Ramban and Hizkuni all on 8:2.
My thought is that the importance of 8:1-3 is that the setting up of the lights/ wicks in the menorah was considered part of the building of the menorah, and this could be why the instructions how to place the lights/wicks were included within the instructions to build the menorah, Shemot 25:37, and not at the end of Shemot 25. With this idea, by Aharon placing the lights/ wicks in the menorah, he completed not only the building of the menorah, but also the construction of the mishkan/ ohel moed since this was the last item in the mishkan/ ohel moed to be built. Moshe had already placed the lights/ wicks in the menorah but the idea of 8:2,3 was for Aharon to set (re-set?) up the lights/ wicks on the menorah to make him the person to complete the building of the mishkan/ ohel moed. 8:2 is a new commandment for how the lights/ wicks were to be set up in the menorah separate from the instructions in Shemot 25:37, and 8:3 records the fulfillment of this new commandment.
The significance of Aharon fulfilling this command, which meant that Aharon completed the building of the mishkan/ ohel moed, was that this act symbolized that Aharon, and the priests who he represented, were the “owners” of the mishkan/ ohel moed, and hence responsible for running the mishkan/ ohel moed. The act of placing the lights/ wicks in the menorah was recorded after the lengthy list of gifts and sacrifices of the tribal leaders in chapter seven to show that the priests were “taking control” of the mishkan/ ohel moed from the people. While the people were connected to the mishkan/ ohel moed though their gifts and sacrifices, they were not the “owners” of the mishkan/ ohel moed and they had no say in the running of the mishkan/ ohel moed.
After the Torah records that Aharon set up the lights/ wicks in the menorah, 8:4 records the law that the menorah was to be made from one piece of gold. Presumably this means that pieces of gold were to be combined (melted) to form a large slab of gold (via a mold), and then the slab of gold would have been chipped away/ hammered, mikshah, to form the menorah. Why was this information recorded here? This information is already recorded in Shemot 25:31-40 and Shemot 37:17-24. Also, 8:3 records that Aharon lit the menorah, so obviously it existed already. Why now was the Torah again recording how the menorah was to be constructed?
Maybe this particular law was recorded in the book of Bemidbar because the menorah with its flower designs was fragile, and this could have led to some of the petals and decorations chipping or even breaking, especially when it was being transported. 8:4 is then telling us that when the menorah needed to be fixed, one could not just add a part back to the menorah. Instead, the whole menorah was to be turned back into a slab of gold and then remade as one unit.
Note, this idea is not obvious. From the instructions in Shemot 25 with regard to the menorah, one might have thought that the menorah only had to be one piece of gold when it was first constructed, but afterwards, if needed, pieces could be added to it. Accordingly, 8:4 informs us that the menorah always had to consist of one piece of gold, and since this law is pertinent only after the menorah was completed, the law in 8:4 is recorded after we read of the menorah being used in 8:3.