The minimum you must know
Saudi Arabia is traditionally a very close alliance for the United States. Saudi Arabia is traditionally a security and economic competitor of Iran. However, Saudi Arabia and Iran have made phenomenal progress on peace with each other after Russia hosted their leadership in Moscow and successfully encouraged the countries to put aside their differences and become friends. There has also been several years of rhetorical outreach from Iran to Saudi Arabia for peace, and more recently from Saudi Arabia to Iran. The United States eagerly supports peace and improving relations between these countries.
The Saudi Arabia & Iran populations peacefully pray together every year at the Hajj, a pilgrimage to Mecca in Saudi Arabia that can attract tens of thousands of Muslims. The populations pray peacefully together and enjoy the holiday even though slightly various views on Islam exist in the two countries. There is therefore every reason for both the populations and governments of the countries to get along. That appears to be what's taking place.
Although the Saudi Arabian and Iranian governments have some competing economic interests, that's true of countless other governments at peace with one another. The leaders of Saudi Arabia and Iran may therefore be ready to achieve open trading and cooperative security relationships. They have made so much progress towards peace with one another that there's great hope around the world that they're becoming real partners and can become lasting friends.