To clarify for anyone who finds this question:
Answer: yes, you need a privacy policy for your application if you use AdMob. Refer to: AdMob Help (bottom of page).
... In addition, you may need to update the privacy policy of your applications to reflect the use of personalized ads (formerly known as interest-based ads) served through the Google Mobile Ads SDK. Please take a moment to review the privacy policies of your apps and make sure they are up to date. Since publisher pages and laws vary from country to country, we cannot offer a specific language for the privacy policy ...
They essentially want you to inform users about the use of targeted advertising. They clearly don't tell you WHAT to write, though.
Personally, I think that should be enough to write something about the fact that you use AdMob to display ads and that AdMob uses the device advertising identifier for personalized ads based on user interests (including the collection and analysis of user data). A link to a Google page to opt out of personalized advertising and Progacy Googles policies ( here and here ) cannot harm.
It also states that you need to have at least a link to your privacy policy in your application and on the store page. But, perhaps, AdMob does not take into account the processing of confidential data, therefore, perhaps a link to the policy on the store page is optional in this case.
As a warning: if you use any Google services (e.g. Location, Firebase, Places Api), you probably need a privacy policy
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