Yesterday at Google I/O, it was announced that Google Assistant was coming to iOS and then later that day it appeared in the App Store for installation.
Google Assistant performs much the same on iOS as it does on Android, with the exception of Google hands-free voice detection. All the great search functionality that is powered by your Google history is present, as well as most of all the other great functions you have come to enjoy on an Android device.
Google Assistant operates similarly to Siri with contextual discussions and some humor. There is not the deep app integration that we see with Siri, but if you use Google products then there may be more relevant history for you on Google Assistant than in Siri.