1. Use AdGuard DNS. https://news.ycombinator.com/item?id=18788410
2. Do not install the app if there's a website equivalent you could use (Facebook, Banking Apps).
3a. Force Stop or Disable apps you use frequently despite web equivalents (Google Maps).
3b. Enable permissions required by apps used occasionally only when in use. Disable them again, once usage is complete (Banking Apps).
4. Use websites on mobile on Firefox with uBlockOrigin/uMatrix, PrivacyBadger, CanvasBlocker, WebRTC blocker.
5. Prefer using 'lite' versions of apps, if you must use an app (Uber Lite).
6. Try to use apps that do not require GooglePlayService or slowly force yourself to (OpenStreetMaps).
7. Use privacy-oriented apps as a replacement to apps that you you use very frequently (Signal, ProtonMail, DuckDuckGo) or use a separate user-profile for those apps (WhatsApp) altogether. https://news.ycombinator.com/item?id=18873433
8. Use LawnChair as your default launcher (or some such privacy oriented launcher).
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Of course there's a big matter of Google services running the show underneath, and you couldn't get rid of that unless you went the microG+LineageOS route. https://news.ycombinator.com/item?id=15617615
Also see:
EFF's Surveillance Self-defense https://ssd.eff.org/en#index
Dumber Phone: https://nomasters.io/posts/dumber-phone/