Quoting Commentary for II Chronicles 7:7

And the Copper Altar … he brought, etc. It is impossible to say that this was the Copper Altar that Moshe made, because it was hidden. It is also impossible to say that his was the stone altar that Shlomo made, and he called it the “Copper Altar,”²II Divrei Hayomim 7:7. because it was impossible to move it from one corner to another without taking it apart,³Alternatively, “he placed the new altar near the Copper Altar, i.e., near Shlomo's altar which is referred to as the Copper Altar. He placed it closer to the Sanctuary than Shlomo's altar without moving that altar to show that it was important to him.—Radak but we learned that the fire that descended in Shlomo's time, did not leave the altar until Menashe came and removed it,⁴Maseches Zevachim 61b. for he demolished the altar, as we find in the Aggadah of [the chapter entitled] “Cheilek.”⁵See Maseches Sanhedrin 103b. I have no way to explain this “[copper] altar” except as [referring to] the basin and the bases of copper, which were accessories of the altar and were near the holy altar. Now, when Achaz made the altar for the idols, he removed the basin and moved it to another side, [farther] away from the Temple, so as not to intervene between the altar he made and the Temple of Adonoy, and he placed it next to the holy altar on the north. We find that our Rabbis⁶Maseches Shabbos 55a. explained a passage in a similar manner, ”And they came and stood next to the Copper Altar.”⁷Yechezkeil 9:2. Did the Copper Altar exist? He said to them, ”Begin from the place where they recite songs before Me.” Thus we see here that copper musical instruments are referred to as a “[copper] altar.”⁸It follows then, that the copper basin which is an accessory to the altar is referred to as “the Copper Altar.”