How do I return actual reference in C++

I am implementing a biconnected-components algorithm. There are several read-only methods written by other in the graph representation that returns a copy. How do I change these to return the actual reference instead of a copy? I am confused. Is there a good source to learn these stuff? Here are some examples:

1. getting neighbors vertex.

   list<Vertex> Vertex::getNeighbors() const {
   return this->_neighbors;
   }
   
   

2. getting edges

   vector<Edge> Graph::getEdges() const {
   unordered_set<string> distinctEdges;
   vector<Edge> edges;
   
   for (auto vertex = _verticies.begin(); vertex != _verticies.end(); vertex++) {
       auto neighbors = vertex->getNeighbors();
           for (auto neighbor = neighbors.begin(); neighbor != neighbors.end(); neighbor++) {
           Edge e(*vertex, *neighbor);
   
            if (distinctEdges.count(e.str()) > 0) {
            continue;
            }
   
       distinctEdges.insert(e.str());
       edges.push_back(e);
       }
   }
   
   return edges;
   }