El / La / Los / Las expressing generality

Whereas in English we say “doctors” to express the general idea embodying the people who participate in the profession, in Spanish “los médicos” is used. This can be confusing for English speakers, as we associate “El / La / Los / Las” as “The”, which is a definite article.

I wonder what the difference is between “nombre” and “sustantivo” to mean “noun”? Are they interchangeable? I like the way “sustantivo” sounds. It’s pleasing to the ear.