You need Sandboxed Google Play installed first. Then you need the Google app, and Assistant app both downloaded and installed, as well as set the Google app as your default digital assistant in Default Apps. (in Settings -> Apps)
That will make Assistant work. You can swipe from a corner of your screen towards the middle of the screen to activate Google Assistant then!
There are a lot of assistant settings to go through, in both the Assistant and Google apps. The Google App is the one you grant permissions to, that affect what the Assistant can and can't do. Ex. Needs SMS permission to send SMS, phone permission to call someone. If your contacts are in your Google account doesn't need Contacts permission, otherwise it does if you want it to send messages or call people by name, not #.
You'll need to allow Assistant to access your watch apps somewhere in the Assistant settings...
You can replace assistant with the Gemini AI as well! Read my other post on this forum, at the end:
https://discuss.grapheneos.org/d/12307-my-early-ride-through-grapheneos-what-works-what-doesnt-and-guide/2