How to have swappable Character Clothing - 3D Model & Meshes / Textures

Here is the code for making an object with a SkinnedMeshRenderer component use another object's rig:

Shared skeleton and animation state

 void Start () {    
      SkinnedMeshRenderer targetRenderer = m_attached.GetComponent<SkinnedMeshRenderer>();
      Dictionary<string, Transform> boneMap = new Dictionary<string, Transform>();
      foreach( Transform bone in targetRenderer.bones )
      {
          boneMap[bone.name] = bone;
      }
      
      SkinnedMeshRenderer thisRenderer = GetComponent<SkinnedMeshRenderer>();
      Transform[] boneArray = thisRenderer.bones;
      for(int idx = 0; idx < boneArray.Length; ++idx )
      {
          string boneName = boneArray[idx].name;
          if( false == boneMap.TryGetValue(boneName, out boneArray[idx]) )
          {
              Debug.LogError("failed to get bone: " + boneName);
              Debug.Break();
          }
      }
      thisRenderer.bones = boneArray; //take effect
  }

In your 3D modelling application, you create one rig and animate it as usual. However, not only the "naked" character model will be bound to this rig, but rather every piece of clothing (at least those that need to deform) will as well, and exported seperately.

In Unity, you then use the script to assign the rig of the naked character object to the clothing object. A database of clothing should be no problem. It can take the form of a Dictionary where string is the name of the item and ClothingItem is a custom class that holds the things you already mentioned, such as the prefab, offsets, etc.