What core concepts should I understand to evaluate gear and build characters in any RPG?

I’ve played many RPGs but I always struggle with understanding gear and stats beyond the basics.

I don’t want build guides or “best items”. I want to understand the general logic behind:

- how to evaluate items

- how damage scaling works

- how to recognize good vs bad stats

- how to build characters without copying guides

What are the core concepts I should learn that apply to most RPGs?