What is app-ads.txt and how do I implement it in AdMob?

First of all, you have to know - What is app-ads.txt?

App-ads.txt is a text file app developers upload to their developer website, which lists the ad sources authorized to sell that developer’s inventory. Just like on the web, the IAB created a system which allows buyers to know who is authorized to buy and sell specific in-app ad inventory, and who isn’t.

How will app-ads.txt be implemented?

Step 1. Provide the developer website URL in your app listing

Ensure that your developer website is updated in the app stores. This website will be used by advertising platforms to verify the app-ads.txt file.

Step 2. Reach out to all ad sources

Get in touch with your direct ad sources and ask for their app-ads.txt line, according to the IAB’s structure below:

Ad source domain, your publisher ID, type of relationship (direct or reseller), ad source ID

Example: ironsrc.com, 1234, DIRECT, 5678

Your direct demand partners should be listed as “direct.” If your partners are using third-party resellers to sell your inventory, such providers should be listed as “reseller.” In any case, you should not add any provider to your app-ads.txt file unless you or your partner have a direct relationship with them.

Step 3. Publish an app-ads.txt file

Create an app-ads.txt file in Notepad listing out all the lines you received, and save.

Step 4. Upload

Upload the file in the root of your domain website (example: https://www.example.com/app-ads.txt).

you don't have to have a website for your app-ads.txt, then you can host it for free on Firebase.

here is a HowTo from Google on how you can achieve this. Publish app-ads.txt with Firebase Hosting

The beauty of this solution is that you have the freedome to add different Urls to any app you have on the Google Play Store.

and on the App page, if the User click on "visite website", you can redirect them to any website you want.