The Google Photos app on mobile devices shows both photos that are uploaded to Google Photos, and photos on the device that are NOT uploaded. Therefore it is dangerous to delete in the Google Photos app: you may delete the device copy while there is no backup. It is recommended to do this in the web app https://photos.google.com/
When you delete a photo in Google Photos it will also be deleted from all synced devices. In practice that are only mobile devices (phones and tablets) since syncing with computers has been disabled.
There are however means to do this:
Disable Backup and sync on the device before you delete from Google Photos. The image gets deleted from Google Photos and other synced devices, but stays on your phone, since Sync is disabled. Also after enabling Backup and sync again, the photo stays on the device and will be shown in the Gallery and File managers, but also in the Google Photos app. The Google Photos app also acts like a Gallery and shows the device copy.
Move the photo to a non-synced folder before you delete from Google Photos. The photo will still be visible in the Gallery and File Managers, but not in the Photos section of the Google Photos app. On Android the photo will still be visible in the "Photos on device" section in the "Library"
Links:
https://www.guidingtech.com/delete-photos-google-photos/
https://www.technobezz.com/how-to-delete-photos-from-google-photos-but-not-your-smartphone/