They haven’t made any military agreements and no sort of alliance or anything like that. But they have sold some new equipment to Iran which is not very surprising given that Israel destroyed pretty much its entire air defences.
But the equipment they sold would only be able to protect small high risk areas and they would also be more effective against the Israelis than the Americans (who have much more resources as well as bases all over the Middle East as well as military vessels and submarines unlike Israel).
For Israel, Iran was barely at the limit of what they could reach with F-16s with extra fuel tanks. But for Americans with in air refueling and their very long distance B-2 Spirit stealth bombers and aircraft carriers, nearby bases etc they are far more flexible.
Checking up on the details there, Russia and China have both sold new equipment to Iran since the 12 day war.
Russia has sold a single S-400 weapon system and Russia has sent some of its HQ-9 air defences.
https://defensemirror.com/news/39946/Iran_Receives_Russian_S_400__Chinese_HQ_Air_Defense_Systems
Russia has also sent half a dozen upgraded Mig-29 fighter jets. It promised to send several of the generation 4+ Su-35s but that is a delayed shipment from 2022 and there is no sign of them yet.
The numbers are small but they would complicate a rerun of the 12 day war by Israel.
https://www.iranintl.com/en/202509236847
https://defencesecurityasia.com/en/russia-an124-iran-s400-su35-deliveries-2025/
More details.
https://theins.ru/en/opinion/antonio-giustozzi/287381
It likely wouldn’t make much difference to the US with its far greater capabilities than Israel.
It is just supply of weapons for a price and doesn’t involve them in any war in any way.