What we call this in the industry is "attribution" and there is a whole sub-industry to deal with this bc it is a complex problem. In short, gaming right now uses last-click attribution so the last advertiser gets credit. FB does influence behavior for sure, though that is difficult to quantify.
Yeah, so Apple are essentially putting themselves in as the end-point of the funnel, which will cause advertisers using last-click to attribute more value to them versus FB/other mobile networks.
"View through" conversions are quite a large area, especially in programmatic. MTA is an important field, but would obviously need to be implemented with some form of in-app tracking tied to your ad providers.