Google Lens made its debut in the Google Photos app on select devices a while back, but now it's available everywhere. You can still search from the Photos app, but Google just added a new shortcut to make it easier to scan your photos. From Lens, just tap the album button at the top of the screen, and you're all set.
The changelog for the latest Lens update (which should have appeared on most devices by now) has a single item that's a bit difficult to parse. Here's the text.
Based on your feedback, we added a new feature. Now, you can search for an item in any of your photos using Lens by tapping on the album icon. If you enjoy Google Lens, please rate the app. Lens is continuously improving, thanks to you.
Lens always opens with a live viewfinder to search for objects in the world around you. However, maybe you already took a picture of something you want to search. Previously, you'd have to go through the Photos app. That's an extra step, and you might not even use Google Photos in spite of how great it is. The new album button (at the very top of the UI as indicated above) lets you pick an existing photo.
The album button opens the Android recent file picker. You can use the navigation menu to select a different source like your Photos albums, another gallery app, or Google Drive. Just find a photo, and Lens swoops in to search it. The results from the image are overlayed as if you'd just captured it in Lens.