Overlaped/intruding meshes in ONE object，how to combine them into one mesh like boolean？ Say I joined multiple objects together （instead of boolean）， the meshes are now overlaping each other. Instead of selecting the faces/verticies one by one， split them into objects and combine them back into one mesh with boolean modifier， is there a smarter way of achieving the same thing（delete concave/intersecring/overlaping faces inside）？
Go to Edit Mode, then press P (mnemonic: seParate) and choose "Loose parts". That'll undo the join, and produce an object for each part of your mesh.