Because the Android can see the other devices. For some reason, the iPhone 12 is not recognized by the Android device. Since everything else will connect to the iPhone 12, that makes the Android device having the problem.
Now you are saying that the Android connects to other devices. Let me make sure I'm still understanding you, because you seem to be contradicting yourself some. You are connecting this Android device to the iPhone's Hotspot correct? And when you mention the other devices, the Android device is able to connect to their Hotspots too, or is it that the other iPhones will connect to the iPhone 12? Or, is it that you are trying to connect the iPhones to the Android device for something. There is a difference. If the iPhone is the device that is providing the service, then the devices are connecting to it.