I believe you're referring to the Image Clipper function, it's made available on One UI 5.1 and on S23 series phones, but they are expanding it to other devices so do keep an eye out.
Once you have the above, it's as easy as 1-2-3:
1. Select image from Gallery;
2. Tap and long hold on the object you want to clip;
3. Release your finger and you will find a few options.
