Q: How does SPN differ from a VPN, in detail? In other comments you said that it's similar to Tor, but what does that actually mean?
If you haven't read these post [0][1] already, please do so and come back with any remaining questions!
[0] https://safing.io/blog/2022/09/06/spn-vs-vpns/
[1] https://safing.io/blog/2020/01/22/how-the-spn-compares-to-to...
Q: Does traffic get routed through multiple nodes before being routed to the destination? If not, what sets it apart from a VPN - apart from being able to choose different servers for every program on your PC?
Yes. And you can configure how much of that you want - per App: https://docs.safing.io/portmaster/settings#spn/routingAlgori...
Q: Is SPN intended to provide privacy against nation-state actors where simply masking your exit node is not sufficient, similar to what Tor claims to do?
"Nation-state actor" is a bit vague, assuming the most common meaning (IMO): a global passive adversary.
Quoting from the Tor design document: A global passive adversary is the most commonly assumed threat when analyzing theoretical anonymity designs. But like all practical low-latency systems, Tor does not protect against such a strong adversary.
Neither Tor, or the SPN (or anything else) will protect you from this. If they can really see everything, they will find you - sooner or later.
What we aim for is to stop exposing you to the data sources of the mass surveillance dragnet: Online tracking and the Internet-network itself.
Q: Who operates exit nodes (and any intermediary nodes in the SPN network), are they owned or rented dedicated servers, and does it include residential connections - to provide unblocking for streaming services?
Currently we rent servers, but will start adding our own dedicated ones once the scale is there. Nodes are also hosted by the community to diversify the node ownership in order to strengthen the privacy protection. Currently not using residential connections.
Q: Is traffic routed through other Portmaster users' internet connections?
No.