How do I backup my photos on Android if I lost my iPhone?

If your iPhone photos were synced with iCloud Photos before you lost your phone, one option is to transfer the photos from iCloud to Google Photos. See: Transfer a copy of your iCloud Photos collection to another service - Apple Support

If you aren't sure whether or not they were synced with iCloud Photos, use any Internet browser and log into www.icloud.com and go to the Photo app there. Photos you see there, if any, are iCloud Photos.