Once you have created an account, by default you are the owner of the account and the Holdings in it.
You can define your User currency (the currency you want your wealth to be displayed in) by selecting Settings on the top right menu of your Home page.
If you have multiple Holdings denominated in multiple currencies, they will be aggregated on the Home page in the currency you have chosen as the User Currency.
The FX rates used to convert your Holdings into User currency are the closing rates for the dates the conversion occurs. This means that if for example you look at your wealth on 14 April 2018, the conversion will have been done at the closing FX rates on 13 April 2018.