So when you make a selection with mask select and hit extract it extrudes the mesh properly, but it also leave the faces that were previosly selected making a new closed object.
The ideal would be to add an option in the settings of the mask that extrude the mesh without creating a new object. or an option to delete /keep masked the faces touching the extruded object (Like Open/closed geometry) by other side the remaining faces of the original extruded object can be deleted easly inverting the mask and with the trim option.
Then because both objects share the same matrix location in some points they are welded once they are joined with the simple merge option.
This option would be ideal to add little details with a very few polygons withouth having to voxel remesh the entire object or go to high poly counts to achieve those results.