API access will be given the all properties that you have added on step 2 regardless of what app you chose while generating the system user token.
As a best practice, and to make management easier, we suggest that you create and use a Facebook App ID that is different from the app/property that's using Audience Network. You can create this specific app id by clicking on Add a New App on developer site. If you need to add the application to your business, you can check out this help center article.
read_audience_network_insightsto be able to read Audience Network insights. Click Generate Token.