That 500AUD doesn’t just sit in an account and magically contribute to anything.
Currency exchange doesn’t actually happen in a vacuum. The only reason your Bangladeshi example is able to send 500AUD to his family, is that someone who has Bangladeshi Taka wants 500AUD to buy goods or services from somewhere that accepts AUD. And there’s a very short list of countries that spend AUD.
So that 500 doesn’t disappear to never return. That 500 is sold to someone who wants to use it to purchase australian exports.