When I see a function or type name, how do I know where to find the interface definition if I do not know what header it is imported with?

More generally, how does one learn about any language intrinsics or standard library functions? A lot of research now is made much simpler with searches. I’ve learned nearly everything by opening a bunch of UFO codes to figure what others are doing; followed by cplusplus.com and other online resources.

Doxygen can help with that OOPS: Main Page - but more generally look for #include statements - these are include files that define the C++ interfaces.