Is it necessary to join the mesh before adding bones? I'm Modeling a Character for animation, but still, I'm not sure about the final result of its shape, so I'm trying to add bones, but still, my character isn't joined in one object, it is still objects parented to each other, and it has the eyelids are connected to lettuce and a controller's bones. so I'm Wondering if it Is necessary to join all the mesh parts together(In one selection) Before I add the final bone controllers. So the point is the body controllers are different from the eyelids controllers, If I joined it I would lose my work There. Plus The mesh is always losing its appearance when I add bones with automatic weights in it. so I think it's a cause of not joining everything in one mesh.
It is not necessary to have all meshes in one object to control it with a single armature. I’ve done multiple objects with one armature many times, Usually because I wanted to preserve two UV maps and did not know yet how to merge them.
Thus, I guess your rigging system and weight painting needs adjustment. There are answered questions to this elsewhere, or you can ask your own new question. As always, I recommend the Blender Manual at docs.blender.org
PS: basically, you create vertex groups that have the same names as bones, and the bones will drag around vertices in those groups. If the vertex is part of multiple vertex groups, it will be controlled by two bones, ending up somewhere between them. This is where weights come in: we can have the vertex have less weight in one group so it will follow the other bone more but not entirely.