How do I stop my iPhone from creating subfolders for photos?

Probably you mean Albums when you say Folders.

Think of albums as playlists, not photocopiers:

• Imagine your iPhone's Photos app as a giant library with all your pictures and videos.
• Albums are like playlists you create within that library. You can add the same song (photo or video) to multiple playlists (albums) without creating extra copies of the song (media file).
• The album itself just holds a reference, a shortcut, to the original file in the library. This saves storage space on your iPhone.

Benefits of this system:

• Saves storage space: By not duplicating files, you have more room for new photos and videos.
• Easier organization: You can add a photo to multiple albums for easy access without creating a mess.
• Keeps things tidy: No need to worry about hunting down and deleting duplicate files.

So, where are the originals stored?

• All your original photos and videos are stored in the main library of your Photos app, typically under "All Photos" or "Recently Added."

Additional points:

• If you want a duplicate of a photo or video, you can use the built-in "Duplicate" function within the Photos app. This will create a completely new copy of the file.
• Starting with iOS 16, the Photos app can help you identify and remove duplicate photos and videos already in your library, further optimizing storage space.

I hope this explanation clarifies how iPhone albums work and why they don't store duplicate media files. Organise and find your photos on your iPhone, iPad or iPod ...