1) Below is my understanding of the scenario you're describing.
i. Advertiser A promotes app B on Google, FB and Apple.
ii. User U is served with ads for app B on Google and FB, clicks on both, though does not engage with downloading app B.
iii. Within less than 24h from the first ad served to user U, the user searches for app B on the App Store, and is served with a promoted ad placement for app B as part of the search results (which also include app B), the user clicks through to download on the search result (and not the promoted placement).
1. In the context the user has not provided permission to track within app B and also within the FB/Google entities, standard IDFA attribution is not available.
2. Apple Search Ads attribution will not be credited with a conversion without the user clicking on the ad.
3. Assuming both FB and Google have utilized SKAdNetwork once the user clicked through, the channel which drove the last click will be attributed with a conversion.
2) StoreKit rendered ad - A framework that allows you to load an App Store product page within the context of the app you're using (essentially without leaving the app to load the App Store app).