It asks afresh. As a concrete example, I've been running the iOS 14 beta for the last week and a bit, Facebook Messenger previously had full access to my photo

It asks afresh. As a concrete example, I've been running the iOS 14 beta for the last week and a bit, Facebook Messenger previously had full access to my photos, and when using it for the first time I got a message from the OS explaining that I no longer needed to grant it carte-blanche access and whether I'd like to deny it access, pick the photos I'd specifically like to give it access to, or let it continue having full access.