(Actually it was a unit mesh but [Mixamo Auto Rigger] could not rig it so I cut hair and clothes then rigged it with Mixamo)
Now I want to join them again. I used Ctrl+J in blender to join the meshes. but it turned to be like this:
@david_bernard_31 thanks for changing topic to be [SOLVED]. I was going to change it anyway but I encountered with an small issue (Solved it right now). and it was :
After joining both mesh the main mesh which had rig on it lost the rigging.(armature animation).
but I could solve it . selected the mesh and hold “shift” key then selected armature then “Ctrl+P” (for parenting) , then select “Armature Deform” . Finally my rigging got back safely .