The more common scenario AIUI for spying is turning someone already in place, either because they have sympathies with your side, or because you have leverage on them - and probably a mix of both. It's a pretty cynical game.
My understanding is spies are mostly compromised nationals as opposed to foreigners trying to blend in.
Yes, but you still need local handlers for them, and they would be “foreigners trying to blend in”
Handlers don't need to blend it nearly as much. They just need a plausible reason to be in the vicinity of their assets. A lot of diplomatic staff are spy handlers. Also, businessmen, military attaches, etc. Every country has some contact with foreigners. Any of those foreigners can be handlers.
Dragon Ladies flew over Soviet Union from 1955 until 1960, when Powers was shot down near Sverdlovsk, i.e. a bit farther to the north from interesting locations. Those spy planes could make a photo of the sites.