Whether you primarily use Siri or Google Assistant (assuming you use a digital assistant) likely comes down to which platform you're invested in—Android or iOS. It doesn't have to be one or the other, though. Google recently updated its Assistant app for iOS to support Siri Shorcuts, which allows users to call up Google's virtual assistant without having to fire up the app.
Once the app is updated, the best way to take advantage of the feature is to record a familiar phrase like "OK Google" or "Hey Google." After you do that, you can tap into Google Assistant without having to open up the app.
It's sort of an end-around solution, though. It's not quite the same as on Android handsets, in that you can't just bark out "OK Google" and have Google Assistant respond. This method goes through Siri. That means having to first invoke Siri, and then saying "OK Google."
That said, it's possible to do all this hands-free, it's just not all that elegant—you would first say "Hey Siri" to invoke Apple's digital assistant, followed by "OK Google" or "Hey Google" (or whatever phrase you recorded).
Obviously this is not the smoothest way to use a digital assistant, but if you prefer Google Assistant when using an iPhone (or iPad), this is an improvement over opening up a separate app.
If you're interested in doing this, update the Google Assistant app or download it for the first time from here. Open it on your iPhone, tap "Add to Siri," then tap the record button and record whatever phrase you want.