Well I haven’t used Nest, my thoughts were for general Node. But yes, listening on :: like that looks ok to me. It sounds like the healthchecks in your first post must be passing now in order to get that app deployed, else it wouldn’t even get to that point.
As regards connecting to it from other apps, yes, assuming they are in the same organization, you should be able to connect using e.g http://app-name.internal:8080. You use http because the connection is already encrypted.
Personally I would experiment with the hostname. Like try using http://app-name.internal:8080. If that works, next try e.g region.app-name.internal. Try and narrow down the failure that way. If e.g the hostname without the region works, but with it does not, that would be one for Fly to investigate.