I downloaded the app on my iPhone and when I click on it, the app doesn’t even open!
Are you connected to WiFi? If not, there is a known bug that can cause the App Inventor app to crash at the first start. To workaround this (assuming you aren't connected to WiFi), you can go to the Settings app on your iPhone, locate MIT App Inventor, and then turn off the switch that says "Show Welcome Screen". The welcome screen and WiFi detection algorithms interact in a way that causes a crash. This will be fixed in the next version of MIT App Inventor for iOS.