Customs at refueling stops?
It depends on the country and may depend on the airport - you have to check the AIP. The term "technical stop" or "tech stop" is often used instead of "refuelling stop", since I guess there could be other reasons than refuelling why you'd need to land and takeoff without exchanging passengers or cargo.

As an example from personal experience flying my 2-seat experimental across the pond, Kangerlussuaq and Narsarsuaq in Greenland do not require crew to go through any customs or immigration checks for a technical stop, while Reykjavik in Iceland does. They let me get out of my drysuit and take a pee break first though, unlike the US CBP!