Q: what is the #1 fundamental driver of long term FX rate between 2 currencies?
Is it PPP? I don’t think so.
Is it supply/demand? Short term yes; long term … this seems less relevant
%%A: “gold” backing.
In the beginning, every pound issued by the British monarch is as good as an ounce of gold (or whatever fixed amount of gold). Therefore anyone holding a pound note can exchange it for the equivalent amount of gold. The monarch then decided to print more pound notes. Logically, all the existing and new pounds devalue. But the country also exports and, in a world of universal gold standard, earns gold. In a more realistic world, What is earned is foreign currency.
I believe Every Singapore dollar issued is backed by some amount of “gold” which in modern context means foreign reserve in a basket of hard currencies. By the way, there’s nothing harder than gold. As the national economy expands, more goods are produced domestically so the CB could issue more SGD, but I guess the CB waits until the goods are exported and foreign currency earned. Such a prudent practice helps ensure every SGD is fully backed by sufficient “gold”.
 incoming tourists also spend their own currency like JPY, therefore contributing to the Singapore foreign reserve. If tourists sell their JPY for SGD outside Singapore, the JPY amounts tend to flow into the global banking systems and back to Singapore banks. I think non-Singapore banks don’t want to go long or short SGD for large amounts, so they would eventually exchange with Singapore banks including Citibank, HSBC, SCB, BOC.
In general, a hard currency is more deeply *backed* by gold than a weak currency. Alternatively, the issuing central bank has other capabilities (besides gold reserve) to maintain the *strength* of her currency. This strength depends on Current Account and economic growth — the 2 fundamentals. Another factor is global competitiveness, but this is often measured by the capacity to export and compete on global markets. After all, it still seems to boil down to earning enough foreign currency to back our own currency — “gold” backing in disguise.