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.

THB to USD conversion is done at the spot rate and the only cost associated is the currency conversion fee, which is charged by the bank, at 0.1% of the transaction amount. This preferential rate, which StashAway has negotiated with the bank, compares favorably against the significantly higher currency conversion fees charged by commercial/retail banks (easily between 0.4% and 1.0% depending on transaction size).