"Infinitely better" is the problem. My d7100 and z5 take amazing pics. They are also the only option for anything requiring zoom. But, for snapshot style pics the iPhone 12 Pro is great, particularly in challenging light situations.
In studio light situations, fstoppers did a video years ago that had the iPhone (6 maybe?) produce pictures that rivaled the dslr at the time.
It's simply more complicated than infinitely better.
Not the person you’re responding to, but lenses don’t improve image quality. All optics have some defects, and high quality lenses will have fewer of them. But a larger, more expensive lens primarily allows for a wider range of aperture and greater control of depth of field. That does allow for more artistic photographs, but objective image quality does not improve.
If you’re still skeptical, consider a pinhole camera with no lens capturing an image at near infinite focus. Adding a piece of glass that refracts the light can’t perform any better than pure parallel rays of light.
Bigger lenses mean more light. More light means better signal to noise ratio. That's an objective metric of image quality.
Phone cameras can try to compensate for this by taking long exposures - actually videos - and warp the frames into alignment to remove blur, but it's a losing game if there's a lot of motion. Or the camera can try and guess the textures and replace them with similar ones, like the Gigapixel AI and similar services do - but then it's starting to move away from capturing the actual scene.
I mean, most of the photos I take are with my phone, because it's the handiest available camera. I don't think people talking about quality from dedicated cameras are trying to say that they're better than phones as a practical tool for taking everyday photos. But you do lose some quality and computational tricks can't get it all back.
Because it is a big claim. And there are many different kinds of photographs you can take in many different situations, so something what is an advantage in one situation is a disadvantage in another.
Computational photography can overcome some of the optical limitations of the small sensor. E.g. you need a really good camera to get the dynamic range that is available with iPhone HDR.
Old camera (I am not aware of any modern cameras having 6mpx sensors) is an old camera. Cheap glass is a cheap glass. The bigger the sensor the better your lens has to be optically, because light is bent at the bigger angles.
The two most obvious areas (ergonomics and interchangeable lenses aside) of advantages of the "proper" cameras with bigger sensors are low-light performance and "bokeh". Otoh, if you want to have as much in focus as possible small sensors win.
No. No matter if you have wide-angle lens or the telephoto lens, you still need to cover the whole sensor with the image and the distance from the last glass element of the lens to the focal plane does not differ that much.
