It’s Apple Review Guideline 5.1.1:
(i) Privacy Policies: All apps must include a link to their privacy policy in the App Store Connect metadata field and within the app in an easily accessible manner…
For Android it’s in their User Data article:
Privacy Policy All apps must post a privacy policy link in the designated field within Play Console, and a privacy policy link or text within the app itself…
This is just talking about developers having to include a link to their privacy policy in the respective field in the App Store/Play Store