It doesn’t always happen that they move in opposite directions. Sometimes they do but not always. Sometimes they move in the same direction, especially for example AUD and NZD, often AUD, NZD and CAD. But these are not constant relationships.
You need to start watching over a period of time which are moving up or down - try some of the heat maps available on the web, or gauge each currency against one major currency such as USD, or develop your own parameters such as which GBP-based charts have rising 50EMA’s etc.etc. You will see relationships fluctuate and this can give you opportunities.