@loschinony With "User" I meant the customer. The pay out currency is the one selected by the customer, not the base one configured in WooCommerce. That is, if a customer switches to Euro, you will be paid in Euro. If he switches to GBP, you will be paid in GBP.
Regarding your other query, there is no "hidden" value. You choose a base currency (e.g. GBP) and enter product prices in such currency, then you can enable additional currencies (e.g. USD and EUR). From that moment, prices in USD and EUR will be calculated automatically, using an exchange rate that you can set manually, or take from Open Exchange Rates.
If you wish, you can also set a price manually in each currency for each product. That is, you can set a product price to 10 GBP, 20 USD and 20 EUR, thus ignoring the exchange rate entirely.