The trade happens through the trade blocs that were put in place.
Step-by-step explanation:
Communist regimes that are not economically liberalized still trade with the world through their trading blocs.
These are governmental institutions designed so as to allow for trade to happen in the country and through the country.
These trade blocs deal directly with the companies of free trade nations where the government in itself trades very rarely as is the case with the US.
Thus the dealing is usually with the government directly.