In short: The Google Camera update for the Pixel 3 has been extracted, and you can now install it on your smartphone to take advantage of the new features that are coming in the Pixel 3's camera. This includes the new user interface as well as RAW support. Now, just because this is the Google Camera app from the Pixel 3, it doesn't mean that it will bring all of the new Pixel 3 features to your older smartphone - like Top Shot. In fact, it doesn't bring many of the Pixel 3 features, including Playground. It's still listed as AR Stickers here.
Background: The Pixel 3 was announced yesterday, and it brought a slew of new camera features, as you'd expect from Google. After all, the Pixel smartphones have always had some pretty impressive cameras, and that is still the case here. Google debuted some new features like Top Shot which essentially continually takes photos while you are taking a selfie, from 1.5 seconds before to 1.5 seconds after snapping the photo. This is so that if someone blinks, it doesn't ruin the picture, as you can still pick the "top" shot, hence the name. It has also improved HDR on the Pixel 3, especially in low-light, with the Night Sight feature, so that you won't have to worry about getting a really dark photo, or ruining it with using the flash on your phone.
Impact: Downloading the Google Camera for your smartphone could break a few things, seeing as it isn't made specifically for that smartphone. But in most cases, it will provide better images than the camera that was built for that smartphone. This is going to work really well on older Pixel smartphones like the Pixel 2 and Pixel 2 XL. Remember when downloading this APK, that you are going to need to turn on Unknown Sources to sideload it onto your device.