What does broad and narrow mean in a general context suppose Broad traits are x, y, z and narrow traits are a, b, c, broad issues are f, g,h and narrow issues are k, j, h? If a trait suppose X cant be further divided and why do we call it broad?
In this context "broad" means generally applicable and with wide (lots of) applications. Examples for broad traits might be "being a good communicator", "being good with numbers", "being tolerant", "keeping situational awareness"
By contrast "narrow", here, means a very specific skill with limited applicability. Examples of narrow traits are "being an expert in the use of commas in 13th century Icelandic poetry", "being able to fix air intake valves on Ford cars made between 2003 and 2005", "being able to remember pi to 1000 digits"
[This is opinion, but you asked for the origin: The usage, i imagine, comes from the metaphor of shallow and deep learning. Imagine a pool of knowledge: shallow learning doesn't go deep into the pool but covers a broad area of the surface. Deep learning on the other hand, covers a very narrow area of the surface but goes deep into the pool.]
Edit answering you question in the comments:
If one were to believe in the type a/b personality theory one could say "There are two broad types of people: type-A people are outgoing and ambitious, and type-B people are introverted". The meaning of "Broad" here is "a rough approximation", or "a view from 1000 feet". Broad in the sense of "Broad brush strokes" made by an artist or a rough sketch.
The antonym of "Broad" in this sense is "Specific" not narrow, so I guess this would be an example you seek: It's not the divisibility that makes something broad, more that it's an approximation.
The expression “streets broad and narrow” is found in the Irish song Molly Malone, which most English-speaking children have probably heard (and sung) multiple times by the time they finish elementary school.
It would be a rare adult that could hear “themes broad and narrow” without mentally segueing into “cockles and mussels alive alive-o”.
A broad street cannot be divided into a collection of narrow streets.
Similarly, a collection of themes might be connected, and apparently haphazard, but still having a common purpose, as a seller of shellfish might have in following a route that sells her wares quickly and profitably.
Edited to add:
Any native English speaker who uses the expression “themes broad and narrow” is introducing a note of whimsy into their text. In keeping with this, we can attempt a thematic analysis of The Three Little Pigs, an English folk tale that also has counterparts in other cultures.
The broad theme of the story is that person will be more successful in seeking their fortune if they work hard to build what they need to protect themselves, since wolves and other predators are ever present. The third little pig that builds his brick house is hard at work laying bricks while his brothers with their houses of straw and sticks are playing in their free time.
A narrower theme is the moral obligation that the third pig has to aid his brothers. In some versions of the story, the first two pigs are killed and eaten by the wolf. In others, they escape, and ultimately run to the house of the third pig, who lets them in without question.
It’s not that this moral obligation is unimportant, but while almost everyone can agree on the need to provide against misfortune, we are less agreed on what we should do to assist those who cannot or will not provide for themselves. It is a narrow theme because it has to be deliberately explored, much as a narrow street in a great city has to be explored with greater caution than the broad avenue where we find others sharing our way.