During the conclusion of the JCPOA in 2015, Iranian negotiators was forced to agree on US demand to a very inconvenient special mechanism, which was enshrined in UNSC resolution 2231. According to it, if even one of JCPOA participant state officially raises in UNSC the issue of significant non-compliance with the terms of the JCPOA of another side, then the UN Security Council should vote not for or against the resolution on the return of sanctions, but for or against the resolution on the extension of the lifting of sanctions. Therefore even the US alone can veto it, and all UN sanctions are automatically returned.
However, to launch this mechanism, it's necessary to be members of the JCPOA, and the United States has withdrawn from it in 2018.
If the United States will be trying to declare themselves a formal participant in the JCPOA, despite leaving it and start this snapback mechanism this will lead to a huge scandal and discredit of the UN. In fact, there may be a situation where the United States will carry out this procedure using it's influence in the UN apparatus and announce the return of sanctions, but Russia and China don't recognize this.