Are all currencies invested in USD?

Yes., for investment products that are invested in US-domiciled and UK-domiciled ETFs, all currencies are converted into USD to be invested as USD.

SGD to USD conversion is done at the spot rate and the only cost associated is the currency conversion fee, which is charged by our vendor, at 0.07% on top of the spot rate. This preferential rate, which StashAway has negotiated with the bank, compares favourably against the significantly higher currency conversion fees charged by commercial/retail banks (which can easily be in excess of 1% depending on transaction size).