The double tap to zoom features is coming to the Google Camera via a new update which should already be rolling out to users of the app. If you have a device which uses the Google Camera then you should be seeing the update live in the Play Store, and if it isn't showing up just yet it should be soon as it seems the update started hitting devices yesterday. It is worth mentioning that this is one feature (though not the only one) which was available to those who were on the developer preview of Android O, more specifically the third preview, so it's not an entirely new feature in terms of this being the first time users are getting a hold of it.
Also worth noting is that this update only seems to be showing up for those who have Nexus, Pixel, or Android One phones, so if you fit into that category then you may want to keep an eye on the Play Store My Apps section or your status bar if you have update alerts set up. As for how the process works, it's as simple as the name of the feature implies. You double tap on the viewfinder when in the camera app in either the photo or video mode and it will zoom in by 50 percent, while double tapping again will zoom back out to the normal setting.
In addition to the double tap to zoom feature, the camera app is also adding another feature which should be useful to users - the ability to switch between the photo and video modes using a dedicated button. First off, you can still use the initial method of swiping to move between the picture and video capture functions, but Google has also now added in a button next to the shutter button for video capture, so swiping won't be necessary if it will be easier for you to simply tap a button to enable video recording. These features are part of the 4.4 version of the Google Camera so if you aren't sure if the update has been applied, as it could have happened automatically if you have this setting enabled, then all you will need to do is check the version number to be sure.