Before, it was possible to open multiple photo in pop up view when you long press and drag a photo to a corner of the screen then back to the center or half of the screen but it was removed after updating to OneUI 6.1, not returned after OneUI 7. It's still possible in documents if opened using Samsung notes document reader.
If you don't know yet, gallery labs will become available if you tap several times on the version number of the gallery app (like what you do in settings for developer options to appear).
