Yes, but if a user doesn’t convert the advertiser will see worse performance metrics as cost per click/conversion/install is higher. That isn’t a good long term outcome for the platform, since advertisers can switch their budgets elsewhere (to Google/Youtube)

There’s a delicate balance, more impressions isn’t always a good outcome. Id assume here given the battle against ad blockers that the platforms do believe the aggregate impression mix leads to good performance for advertisers though

Partially, yes. Ads are also psychological, and tracked cross-platform. If you see a BMW ad on Facebook, drive by a BMW billboard on the highway, search BMW on Google, and then go buy a BMW, they have ways of fractionally allocating the conversion to the different impressions. It's pretty crazy.

I always thought last-click attribution was generally used even though partial attribution was technically possible. But my info is a few years stale.