The feature is only available on Google Photos v 5.22 and above.
You cannot decide what photos come in the memories section of the app.
You can set it up from the wallpaper section of your phone.
The new update from Google Photos is a boon for people who are bored of their wallpapers. The new feature lets users set a dynamic wallpaper from their own catalogue of photos.
Google Photos version 5.22 and above has the feature included, building on the Instagram story like memories field, the main catch being Google defines what counts as a memory. You can learn more about how it works on Google’s blog post.
How to use Google Photos Memories as a dynamic wallpaper
To enable this feature, you have to first update the Google Photos app to version 5.22.
Then just like setting a live wallpaper on your Android phone, press and hold the home screen until the wallpaper menu pops up.
In the live wallpaper option, you will be able to see the ‘Memories’ tab there.
This feature will cycle your memories as the background of your phone.
There is an option to ‘Open Photos’ on the preview page, but you won’t be able to customize the selection. It would just open the Google Photos app.