https://wccftech.com/download-google-camera-mod-that-adds-tracking-autofocus-motion-metering-in-night-sight-on-the-older-pixels/

Unlike the Google Pixel 3’s hardware, its camera specs were not included in any of the slew of leaks that preceded the devices’ release. Apart from the quintessential Stock Android experience, one of the biggest selling points of the Pixel line of devices has always been their cameras, and the Pixel 3 is no different. Most of the heavy lifting is done via Google’s machine learning prowess via the Google Camera app, which works in tandem with the hardware to offer an unparalleled camera experience. However, the older generation Pixels can’t use some of the newer features in the Google Camera that includes tracking autofocus and motion metering.

Tracking autofocus allows you to select a subject which will be kept in focus throughout by locking on to that object and track it through the scene. The feature comes in handy in scenes where an object is constantly moving, for example, a kid or a pet. Thanks to an XDA Developers member, it is now possible to enable tracking autofocus on the Pixel and Pixel 2 (XL variants included).