You can install Collabora Office from Google Play Store or Aurora Store. Google Play Store is probably the most secure way of installing the app, with advantages like automatic updates.
If you prefer to instead, you can download the APK from developer website and install it on your GrapheneOS device. You can choose the latest arm64 version.
Installing APKs from a website works in just the same way as on other Android devices. Being able to do that doesn't have anything to with the Google Tensor.