I want to delete a photo from my photo library but keep it in folder

Open your Photos App and locate the photo(s) that you wish to save to a folder.

At top right of the Photos App windows, tap  Select. Individually tap the thumbnail icon of each photo that you wish to save elsewhere; a blue tick will appear on each photo that you select. Tap a photo again to deselect if you make a mistake.

When you have selected all the photos that you wish to save to a folder, tap the Share button (a square with an upward-pointing arrow). The Share menu will appear; from here, select Save to Files. Navigate to where you wish to save the selected photo(s), creating a new folder if required - then tap Save.

After saving your selected photo(s), should you choose to do so, you can delete the original from your Photo Library.

All of your photos are stored in the library. If you delete it from there it is also deleted from all albums and folders. Note that it doesn't take more storage; there is only one photo stored and it is displayed in multiple places.

You’re very welcome.