You’ll find a list of subprocessors Fly uses here: Fly.io Sub-processors · Fly Docs
(and then you’d have to find subprocessors for those and so on)
Or… if you are on the scale plan, you can email support and they might be more open about the underlying specifics.
See also: Set/get outgoing IP address for whitelisting on external APIs/Hosts - #12 by ignoramous | Outbound IP from a certain region - #2 by ignoramous