1. Reduce your apps to the minimum required
2. Turn off location services, unless you're using that app
3. Turn off notifications
4. Turn off Bluetooth
5. Turn off Siri in ALL instances
6. Disable any app "reporting" features, including back to Apple
7. Use Firefox as your default browser
8. Remove all Google & Microsoft apps
...
Off the top of my head: I'm sure others will have a lot to add.
@chessert That's good advice! Thank you for making the list! :)
Not just my advice: that's what I do. I'm an Apple iOS/MacOS/Linux user by informed choice.