There's also "dynamic DNS", which is basically just caching one side of that server/relay/TUN/STUN handshake, and relying on DNS for global discovery.

For Iroh vs Scuttlebutt / DHT, I'll break that into two parts:

1) Iroh uses DHT for host discovery: https://docs.iroh.computer/about/faq#how-is-iroh-different-f... , and Iroh is more about "use that DHT to get a usually-direct connection globally and then you can do whatever you want", while Scuttlebutt is strictly "... and use that connection to exchange append-only logs via gossip, to implement the Scuttlebutt protocol". (Iroh does have some first-party protocols you can use, but it's lower level in general)

2) Scuttlebutt isn't DHT-based, it's "connect to a known IP to get its data and discover connections" -> "connect to them and repeat..." -> "connect further..." -> etc, plus limited-hop feed replication by default. There isn't a global lookup to connect to any member or retrieve any data, it's all friend-of-a-friend connections and you can (and do) lose connection to someone if they get a new IP address and there's no F-o-a-F(-o-a-F(...)) replication route from them that reaches you (rare in practice since they likely re-connected to people they follow, which eventually trickles data through the mesh similarly to before). This is also part of the reason that it works instantly when you're on the same network as someone - it's less "it can work locally if you don't have internet access" and more "local is just a discovery method, the internet isn't special at all because it's all just direct connections".

And as far as I understand Nostr, it's conceptually similar to Scuttlebutt, but with direct support for centralizing for performance (relays) and some degree of mutability / forgetfulness / etc. Scuttlebutt is a bit extreme about its logs being immutable and the only way to exchange data, and it's part of the reason it can have rather major perf issues (like needing to pull gigabytes of data before you can discover a feed's display name). (I say this as a fan of Scuttlebutt in principle, but not in practice - there are lots of practical issues with existing implementations that could be solved, but haven't, and it's a large part of why the ecosystem split into other protocols)

It may also be worth pointing out that DHTs also need stable hosts to serve as initial bootstrappers, and apps that use them tend to hard-code a web URL where they can get a small list of those nodes. They just use them to discover other nodes, and save them for next time so the bootstrappers aren't constantly needed.