“Do you understand what “decline” means in this context now?”
You still did not define “declined”. You defined exchange rates.
I showed you how the decline in relative purchasing power evidenced by the change in exchange rates.
verb
1. (typically of something regarded as good) become smaller, fewer, or less; decrease
The purchasing power of every BRICS member’s national currency has declined against the U.S. dollar.
What part of this confuses you?