Google Photos' video capabilities often feel like a second-class citizen compared to still pics. While the app offers dozens of editing options for photos, you can only rotate or trim videos, and their playback experience is quite lacking. Skip and rewind buttons were added a while back then disappeared, and there are no controls to speak of, just a play/pause button. Another missing feature is the ability to zoom and pan in a video, but that is now starting to show up for some users.
If the feature is available on your device, you'll notice that you can double-tap to zoom any video so it fills the screen. Double-tapping again brings you back to the zoomed out view. You can also pinch to zoom, then move the zoomed video around to focus on a specific area. See it in action in the video below — look for the round white tap pointers to see how I'm interacting with the video.
The feature is rolling out as a server-side switch and doesn't seem to require a specific Google Photos version; I have a Pixel 4 XL and Pixel 5 running the same v188.8.131.523915353 of the app and logged in with the same account, but only the Pixel 5 can zoom in videos. On my Pixel 4 XL, no matter how much I double tap or pinch, nothing happens. I tried updating the app to the newer v5.28 release (APK Mirror), but that didn't force the option on. Your mileage may vary.