"Cursor to Selected" Puts 3D Cursor to the World Origin Instead of Center of the Object (STL File) This is an STL file I imported to Blender for 3D print. I have similar problem when dealing with most STL files (downloaded from Thingiverse and a like).
I have 3D cursor sets in the middle of selected edges like below:

If I ⇧ Shift +  S then select Cursor to Selected while having the main model selected, the 3D cursor will be positioned like:

How can I make so that my 3D cursor positioned in the middle of the model?
Additionally, when I imported the STL file, the default position of the model is always not in the World Origin. So, that's why the model isn't positioned in the World Origin.
You need to recalculate your object origin.
select your object and go to object &gt; set origin and pick one you prefer.

most imported assets, export with 0 for object origin to avoid unwanted problems.