Improved version of https://gist.github.com/simonwittber/0297a2a8127efddd818d624ce14bd6a8 . With this one, you can extract all sub meshes (inside a Combined Mesh) which be used in a GameObject.

UnitySubMeshesExtractor.cs

public static class UnitySubMeshesExtractor
{
    public static Mesh ExtractSubMesh(Mesh m, int meshIndex, int count, Transform meshRenderHolder)
    {
        Vector3 pos = meshRenderHolder.position;
        Quaternion rot = meshRenderHolder.rotation;
        rot.y = -rot.y;// correct the translation calculation
        
        var vertices = m.vertices;
        var normals = m.normals;
        var uvs = m.uv;
        var newVerts = new List<Vector3>();
        var newNorms = new List<Vector3>();
        var newUVs = new List<Vector2>();
        
        int totalTris = 0;

        var allTris = new List<List<int>>();
        

        for (int iterMesh = 0; iterMesh < count; ++iterMesh)
        {
            var triangles = m.GetTriangles(meshIndex + iterMesh);
            
            var newTris = new List<int>();
            
            for (var i = 0; i < triangles.Length; i += 3)
            {
                var A = triangles[i + 0];
                var B = triangles[i + 1];
                var C = triangles[i + 2];

                newVerts.Add(rot * (vertices[A] - pos));
                newVerts.Add(rot * (vertices[B] - pos));
                newVerts.Add(rot * (vertices[C] - pos));

                newNorms.Add(rot * normals[A]);
                newNorms.Add(rot * normals[B]);
                newNorms.Add(rot * normals[C]);

                newUVs.Add(uvs[A]);
                newUVs.Add(uvs[B]);
                newUVs.Add(uvs[C]);

                newTris.Add(totalTris++);
                newTris.Add(totalTris++);
                newTris.Add(totalTris++);
            }
            
            allTris.Add(newTris);
        }
        
        

        var mesh = new Mesh();
        mesh.indexFormat = newVerts.Count > 65536 ? IndexFormat.UInt32 : IndexFormat.UInt16;
        mesh.SetVertices(newVerts);
        mesh.SetNormals(newNorms);
        mesh.SetUVs(0, newUVs);
        
        mesh.subMeshCount = count;

        for (int iterMesh = 0; iterMesh < count; ++iterMesh)
        {
            mesh.SetTriangles(allTris[iterMesh], iterMesh, false);
        }
        
        mesh.RecalculateBounds();
        
        mesh.name = m.name + "__" + meshIndex + '_' + count;

        return mesh;
    }
}