Equities and (listed) commodities are relatively easy to get a handle on but it genuinely takes months / years even at the frontline to understand how fixed income and FX works because its still almost entirely an OTC market. There is more central clearing than ever before but e.g. if I (say) buy a bond, fund it using a repo, swap my loot back some other currency, quite a lot of this could easily still be relying on humans pressing buttons and wiring money around.

To learn how and why these things are traded, however, read this book, the only (good) truly beginners guide to fixed income:

https://www.jdawiseman.com/books/pricing-money/Pricing_Money...