[alxlaz]

Oh boy. I still have nightmares about this. At $former_workplace we had an entire SDK, a few hundred thousand LoCs in total, with basically no documentation whatsoever. The team that wrote most of it had long been laid off. We traded notes on how to do various things but as soon as you went out of whatever module you'd been typically working on, all bets were off.

I don't know much about Angular but I think most of these things are pretty much universal:

> A common issue I face is when I want to import a module( and know that the functionality exists) but don't know what do I call and where can I call it from

Generate call graphs from the source code. It's generally a good bet that the functions at (or near) the top of the call graphs are the ones that you're supposed to call.

If the library has automated tests, have a look at those -- it won't give you much information about idiomatic usage, but will at least tell you what parts of the whole thing you're supposed to interface with.

Liberally grep through the source code for whatever functionality you're looking for. In the absence of documentation, you'll have to create your own "mental map" of what things there are, and where.

Other than that, all I can do is recommend everyone else's generic advice: read the source code, take lots of notes.