General flex nib compatibilities between brands for a newbie

I'm a fountain pen enthusiast, but new to flex nib fountain pens (I love my chepo flex nib dip pen though) and cannot figure out how to determine nib and nib-housing compatibilities between brands and aftermarket products.

It's clear that brands like Lamy are doing their own thing, but I wonder if there are some identifying characteristics, a rule of thumb or some general grouping that is not immediately obvious to me about the pens and their interchangeablility that I keep reading about (here as well).

As and engineer, I was expecting a sizing/numbering system or types to be thrown around but it is always about using specific x or y nib or upgrading so and so pen, but on the product sheets none of the tech specs indicate any kind of compatibility (other than brand specific replacements).