The first is dividing a space equally into one rate, and equally into another rate, and playing both together.
For example, play 8th notes with one hand against triplets with the other hand. You’re doing this without thinking of any smaller subdivisions.
The best way to figure out how polyrhythms should sound is to think of a common subdivision.
Check out this video to see how it applies to 2 vs 3
We can hear any polyrhythm (X vs Y) in PolyNome by playing every X notes of a subdivision with one sound and every Y notes of the same subdivision with a second sound.
Check out this video to see how to hear 4 vs 5 using this method.