most teams spend a lot of time thinking about user authentication. who can log in, how, with what credentials.
and then the services behind the scenes just... trust each other. because they're on the same network. because they're in the same VPC. because nobody questioned it when the architecture was first drawn up.
My favorite place is Rovinj in Croatia. The old town is magical. As for the worst, I don't have one. very experience brings new perspectives and growth.