For the ordering process, either in the order manager or an ecommerce front end, the currency is set in the product store with the ProductStore.defaultCurrencyUomId field. It defines the currency used while ordering. The currency is set in the shopping cart from this value at the start of the ordering process and then used thoroughly during the ordering process. So OOTB (Out Of The Box) an order (and the following processes, invoices, etc.) is always in a sole currency.
In other words, you can defines product prices in different currencies but to use these different currencies you have to set different stores with different currencies.
There have been recently an effort to use a store with multi-currencies, it's a good example of what should be avoided. See - OFBIZ-5767Getting issue details... STATUS and related issues and related commits comments of all related issues.
Note that in the order manager you can force the currency (override default store setting) in the 2nd screen. There is no such possibility in the OOTB ecommerce frontend, but reuising the order manager way should not be an issue. There are then many possible scenarios and it's better to let OFBiz user to choose their own.