Cracking a CHAP from Freeradius
There is no easy and quick generic way to learn things like this. It's knowledge you amass over months and years by reading, observation and experience, combined with a fair amount of common sense.

In case of hashcat the wiki and forum are valuable resources. For generic knowledge about cryptography and computer science wikipedia is a nice place to start (although it may sometimes be too technical for a beginner).

Be curious and read articles about whatever seems interesting.
Also a good idea is to read the RFC docs , even if they are not easy to understand. They are our basic docs if we add new functions to our tools:
where XXXX is the RFC number of the doc
for example: