The UV detach polygons is different from the Modelling function “Detach polygons” which actually separates the mesh at that point.
The UV Version doesn’t. so there’s no need to attach them. they’re still attached.
If you apply a different mapping type, it will effect the entire model.
It’s like the Seams and Pins for UV unfolding,. It only effects the UV View of the model, so if you change the seams or change the mapping type it all goes back to normal.
Hope it helps