How importers and exporters sell/buys products if their country currency is non convertible or when non residents want to invest in the country? For example in Brazil.
Loosely speaking, by first getting permission from the relevant regulatory authorities. Here is some color on BRL from one bank's EM FX guide:
"[spot] FX transactions must be registered with the BCB and booked against a financial institution authorised to deal FX by the BCB...Interbank transactions between Brazilian financial institutions can be booked over the counter (OTC) or through the BM&F Bovespa Foreign Exchange Clearinghouse (BMC)...Non-interbank spot FX is available for any currency pair formed by BRL and a convertible currency. The FX transaction must be linked to an authorised underlying transaction by way of documents filed with or at least readily available to the authorised financial institution with which the FX transaction is booked"