It's literally a 5 minute Google search.
The UK and Denmark joined before the Euro was a thing, and thus, were allowed to stay out. No other country that joined after 1999 ever got an opt out, because this is not legal anymore.
not a metaphor for anything
Iceland has a small economy and its biggest trade partner is the EU, they're already a member of EFTA. The Euro would benefit them more than harming them, the real issue was always fishing rights and legislation