Fundío' is a colloquial and shortened form of "fundido," which means "melted" or "fused." It's an interesting word because it's used in a non-literal, slang context.
In the song, "Fundío' con el piquete del prieto" translates to something like "Fused with the style of the dark-skinned guy." Here, "fundío'" implies being completely immersed or integrated with a certain style or swagger, suggesting a strong identification with a particular aesthetic or attitude.
iPhone is a taunt, delivered over a Dominican dembow beat and a sampled English hook that keeps circling back like a nursery rhyme.
The whole song hangs on one image: a phone that gives everything away. Luis Brown tells a rival to go and check his girlfriend's iPhone, because the photos and the videos on it are of him. From there the boasting widens out. He mocks the other man for crying over a woman who was never really his, he points out that the guy's own friends are robbing him, and he insists nobody in his own crew folds for small money.
The second half turns to what he has earned. Three years of getting paid, a house that is fully his, a net worth going up. The drugs and the swagger sit right next to that, which is the point. This is street bragging in Dominican Spanish, dense with local slang and dropped consonants, and the sampled English chorus keeps floating over the top of it as if nothing were happening at all.