They must produce light or we wouldn’t be able to see them. Jupiter does not produce its own light but reflects light from the sun.
We see brown dwarfs with infrared telescopes. The same with red dwarfs. Red dwarfs make up something like 80% of the stars in the universe.
Red dwarfs do emit visible light but they’re pretty dim so not one is visible with the naked eye from earth. The nearest is Proxima Centauri at 4.5 light years.