These buttons are on the subtool tab of your tool pallet.
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The have been in Zbrush for quite a while (as long as Subtools have been around) to my memory.
Polymesh3D’s and skins are more or less the same thing–contiguous meshes.
Primitives (Sphere3D, Cylinder3D, Gear3D) and Zspheres have different properties until they are made into polymesh’s by way the Make Polymesh3D or Skinning operations.
Merge visible simply puts all the mesh’s into the same tool (and optionally welds identical points.)
Insert Mesh should insert the source mesh into the target, but I do not use that brush much myself, preferring to merge subtools once I have the subtools all arranged the way I like. (You don’t have to do it this way, it’s just the way I fell I have optimum control of positioning.) Insert mesh is good when you have many repeated inserts of the same mesh, like bullets in a bullet belt. It works by tangents rather than absolute controls.