The media always hypes things more than their actual size. Most likely there won’t be a regional war, it is too expensive for everyone to go into war. And one side is stronger than the other. But if it does happen, here is how I think it might look like: Israel(most likely) or Iran/Hezbollah declare war on each other. They advance their troops to the border for invasion. International pressure rises to prevent a war. If pressure fails, aggression starts heavily. You can expect Israel to attack Iran where it hurts(it’s milita leaders or Tehran directly) and expect Iran to do the same(attacking Israeli citizens with missiles). Every country in the world has to pick a side to support. West would support Israel and supply it with ammo. Iran might get support from North Korea, possibly Russia and China(although they are not strong allies of Iran). I expect Arab countries to not join the war, or join lightly(sending observation force, or some weapons through unofficial means). Expect Iran’s militia’s in Syria, Lebanon, Yemen, and Iraq to provide varying degress of help to Iran. Most effective would be Hezbollah which is it’s strongest and it’s closest to Israel. Second would be Syria, it could become a front on the Golan heights, Iranian milita has strong presence there. Iraq/Yemen won’t provide much, likely only used for enforcement(securing Red Sea, land route from Iran to Lebanon) Expect destruction of Lebanon’s villages, refugess from the Middle East fleeing the war and the economic situation.
Take Simon with a grain of salt, but this Warographics on What a war might look like along side the context of RealLifeLore background on US, Iran relations might give some further understanding if you are after some short intros.