Not even necessarily high-end cameras -- just any camera with a lens capable of a physically larger aperture. Buy the cheapest used Canon Rebel or whatever dSLR you can find, open the lens up to f1.8 or whatever it's capable of, and you've already got something with way more flexibility and light-gathering ability than the best iPhone camera. The iPhone is going to win over a low-end/outdated dSLR in image processing and maybe even sensor pixel count -- but it can't compete on optics because the lens is just too small.
iPhones have incredible cameras for their size, but comparisons to dSLRs and good mirrorless cameras are totally apples-to-oranges (pun intended, sorry).
You are aware the aperture on the iPhone is f1.8, which is pretty dang good. You aren't going to get that in any kit lens, you're going to have to spend some money on a new lens. Granted you can pick up f1.8 50mm lenses for pretty dang cheap, but you know the old adage you get what you pay for.