Who upholds international law?

The Council of the United Nations

"Ultimately, the Security Council of the United Nations is empowered to broadly enforce international law. They can do this through sanctions, peace-keeping operations, or formal censures."

https://www.allamericanmun.com/how-is-international-law-enforced/#:~:text=Ultimately%2C%20the%20Security%20Council%20of,keeping%20operations%2C%20or%20formal%20censures.