You do not appear to have heard of dynamic currency conversion.

The situation is where the invoice is originally in local currency but you are offered an immediated conversion to the home currency of your card. Invariably, it is at a very poor rate of exchange that you should decline. In theory, you should be offered the choice between the billing currency of the merchant and the home currency of your card although some unscrupulous merchants do not always do so.

The Cambodia example cited above is a different situation, where the billing is in USD to start with, as sometimes happens in some developing countries. There is therefore no currency conversion as such as you are billed natively in USD. If anything, there would be a conversion to local currency if you wanted to pay cash in local currency