Mildly Infuriating

Oh cool, there's a 200mp camera. Something that only pro photographers care about lol.

Oh this is a fun one! Trained, professional photographers generally don't care either, since more megapixels aren't guaranteed to make better photos.

Consider two sensors that take up the same physical space and capture light with the same efficiency/ability, but are 10 vs 40 megapixels. (Note: Realistically, a higher density would mean design trade-offs and more generous manufacturing tolerances.)

From a physics perspective, the higher megapixel sensor will collect the same amount of light spread over a more dense area. This means that the resolution of the captured light will be higher, but each single pixel will get less overall light.

So imagine we have 40 photons of light:

More Pixels    Less Pixels
-----------    -----------
1 2 1 5         
2 6 2 3         11  11
1 9 0 1         15  3
4 1 1 1         

When you zoom in to the individual pixels, the higher-resolution sensor will appear more noisy. This can be mitigated by pixel binning, which groups (or "bins") those physical pixels into larger, virtual ones—essentially mimicking the lower-resolution sensor. Software can get crafty and try to use some more tricks to de-noise it without ruining the sharpness, though. Or if you could sit completely still for a few seconds, you could significantly lower the ISO and get a better average for each pixel.

Strictly from a physics perspective (and assuming the sensors are the same overall quality), higher megapixel sensors are better simply because you can capture more detail and end up with similar quality when you scale the picture down to whatever you're comparing it against. More detail never hurts.

... Except when it does. Unless you save your photos as RAW (which take a massice amount of space), they're going to be compressed into a lossy image format like JPEG. And the lovely thing about JPEG, is that it takes advantage of human vision to strip away visual information that we generally wouldn't perceive, like slight color changes and high frequency details (like noise!)

And you can probably see where this is going: the way that the photo is encoded and stored destroys data that would have otherwise ensured you could eventually create a comparable (or better) photo. Luckily, though, the image is pre-processed by the camera software before encoding it as a JPEG, applying some of those quality-improving tricks before the data is lost. That leaves you at the mercy of the manufacturer's software, however.

In summary: more megapixels is better in theory. In practice, bad software and image compression negate the advantages that a higher resolution provides, and higher-density sensors likely mean lower-quality data. Also, don't expect more megapixels to mean better zoom. You would need an actual lense for that.