Once the update has been installed, Assistant users are presented with an "Add to Siri" button that lets them record a phrase of their choosing to be used after saying the "Hey, Siri" command. As long as the iOS device is unlocked, users will then be able to access Assistant by combining the two commands.
The Siri Shortcuts feature also supports additional phrases that users often use with Google Assistant. As The Verge notes, this opens up possibilities for creating custom voice commands for Google actions, including smart home routines, from within Siri.
This isn't the first incidence of virtual assistant crosstalk on Apple devices. Last year, Amazon and Microsoft announced a collaboration that would allow iOS users to access Alexa via the Cortana app, with a "Hey Cortana, open Alexa" voice command.
However, in the latter case, iOS users still can't say "Hey Cortana, open Alexa" from the Home screen, or when the device is locked, because Apple only allows third-party virtual assistants to function within their apps.
Google's Assistant app is a free download for iPhone and iPad, available on the App Store. [Direct Link]