One chart explains most of what your pairs are doing
The US Dollar Index (DXY) measures the dollar against a basket of six currencies. The weights are fixed and heavily skewed:
| Currency | Weight |
|---|---|
| EUR | 57.6% |
| JPY | 13.6% |
| GBP | 11.9% |
| CAD | 9.1% |
| SEK | 4.2% |
| CHF | 3.6% |
Read that first row again. DXY is nearly 58% a euro trade. It was constructed in 1973 and the weights have barely changed since, which means it under-represents China, Mexico and Korea entirely — the US's actual largest trading partners today.
That is not a reason to ignore it. It is a reason to know exactly what it measures, because most traders treat it as "the dollar" when it is closer to "EURUSD, inverted, with some noise."
What it is genuinely good for
The dollar is on one side of most retail pairs. If DXY is trending up, being long EURUSD, GBPUSD and AUDUSD simultaneously is one bet against that trend — which the correlation lesson covered.