Hello,
I believe the iPhone feature lifts the main image off the background, essentially leaving blank space around it which can be shown differently on different platforms and apps. You may want to copy the image then paste it onto a black background then share it.
Lift a subject from the photo background on iPhone - Apple Support
"Lift a subject from the photo background on iPhone
In the Photos app , you can isolate the subject of a photo from the photo background, and then copy or share it in other documents and apps.
This feature is available on supported models and can also be used in Safari, Quick Look, and more.
- Open a photo.
- Touch and briefly hold the subject of the photo. When it’s outlined, do one of the following:
- Tap Copy, then paste the subject into an email, text message, or note.
- Tap Share, then choose a sharing option, such as AirDrop, Messages, or Mail."
I like to use the Preview App on my Mac for these type of photo edits as its more versatile with its available features.
Extract an image or remove a background in Preview on Mac - Apple Support
"Extract an image or remove a background in Preview on Mac
You can extract part of an image. For example, you can extract one person’s face from a group photo and then paste the face elsewhere.
If an image’s background consists of areas that are mainly the same color, you can also remove the background."
I hope this helps.