There are no doubt that the United States is the largest arms exporter in the world and the majority of their revenue is derived from the sale of arms, which contributes to the growth of their economy. If there is no war, who do they sell weapons to, so I also agree with you that America is the mastermind of most of the wars in the world. No one benefits more than them, one way or another they are always manipulating all that chaos, unfortunately Ukraine has become the target this time.
This was is an exception because the benefit of US in this war is not monetary as we know they haven't been "selling" weapons to Ukraine but "donating" it with a huge billion dollar budget dedicated. Also this conflict is causing high inflation in United States which is damaging their economy. There is also a secondary damage as US dollar is being dumped as reserve currency slowly but surely.
The benefit is mostly geopolitical for US as every day Russia is stuck in Ukraine it is increasing the cost for Russia. Every asset they lose, their military becomes 1 asset weaker. This is why US is all for prolonging the conflict.
It also weakens Europe and increases their dependence on US while keeping the dumping petrodollar alive.