Which currency is preferred when countries with different currencies purchase imports/sell exports?
There isn't a typical answer.

The vendor may want paying in whatever currency represents the highest proportion of their costs, as this reduces their currency risk.

But there may also be restrictions on what currency they can trade in; how much of some particular currency they're allowed to hold; there may be capital controls in place; some currencies might be easier to hedge than others; trade in some currencies might attract that country's tax authorities, if significant movements have to be registered; ... and so on.

They might want to do business across various buyers in a basket of currencies, as a way to minimise currency risk through diversification.

They may prefer to do business in currencies with a higher liquidity (USD, GBP, EUR, CHF, AUD, JPY, CAD).

There are lots of variable factors in play. And it's a negotiation between vendor and buyer; the above is written as if the vendor holds all the power, but similar issues apply to the buyer too. For example, the buyer may want to pay in whatever currency represents the highest proportion of their income.