It is a stand system for making macro photos with S24 ultra. It may fit other smartphones too (Note 10 plus is at the cover picture).

The recess at the slider is to optionally apply some frog tape or whatever to get more grip to the phone or level it if required.

You need some scews in addition to complete:

1 x M5 x 30-40 mm hex socket screw full threaded for clamping the dovetail

3x m3 x 18 mm hex socket screw for fixing the holder

OPTIONAL:

2 x M6 x 30-40 mm countersink screw (for travel kit only)