not sure if this is right but i'm guessing they have arrangements for their payment processor based overseas in each of these countries (how exactly is perhaps an industry secret, especially when they are themselves Instarem a remittance company for years), and charges are 'charged' there so it's in the local currency hence they can avoid forex, but Amaze will 'slight' markup in each transactions which is where their margin comes in.
for local charge in local (SGD to SGD), there don't seem to be any markup and benefit for them in doing so (unless to sell user behaviour data to 3rd parties for consumer behaviour analytics purposes) which is perhaps why they've decidedly move to no longer give cashback aka insta points from 1 jul onwards given it's $ given out earlier during last year launch (Jul 2021) while travel is still not prevalent to encourage more take up rates of the Amaze card. Most new companies likes to dangle carrots during launch (rem Uber/Grab fight at the start, similar to Shopee, Lazada, etc) and slowly cut back once they have enough market penetration to cut back on the carrots which are deemed losses at their founding years.