Whats mean 'Note that using non-const operators can cause QVector to do a deep copy.'?
Whats happen at every Qvector[i] ?
Qt Containters use Implicit Sharing. When you need a non-const reference it needs a deep copy

If my case mw3d->push_back(*w3d_world); Iam storing pointers ins`t it ? So , what kind of copy are going to be made ?
Wrong, you're storing the value of pointer (*w3d_world)