Google release hints at this being more than just Siri:

> Apple and Google have entered into a multi-year collaboration under which the next generation of Apple Foundation Models will be based on Google's Gemini models and cloud technology. These models will help power future Apple Intelligence features, including a more personalized Siri coming this year.

... https://blog.google/company-news/inside-google/company-annou...

This is actually the most important part of this announcement, and excellent news. I was pretty disappointed that they were going with an existing player rather than building their own models. But this implies that they will continue to build their own base models, just using Gemini as a starting point, which is a pretty good solution.