Where can I find information on a large number of "exchange" or "trading platform" APIs showing which currencies (digital and fiat) they convert between?
Since you originally stated that you wanted to write a trading bot, perhaps you will find the following library useful:
"XChange is a Java library providing a simple and consistent API for interacting with over a dozen Bitcoin exchanges providing a consistent interface for trading and accessing market data" (according to their GitHub page)
Unfortunately, I cannot tell you more about it, as I just stumbled upon it by accident.
Each site has its own set of apis and documentation, I don't think it's likely that you'll find a single site with a complete set of APIs.
Here are some links to get you started: