Luuggage is not weighed at check-in unless there is something unusual about the route, weather or a/c type. It is for all intent and purpose given a notional weight. For example BA notional weights are 12kgs UK 14 kgs EU and 16kgs World. They have used these figures for the last 10 years and you will no doubt be surprised at how low they are.
Bins / containers are also given fixed weights dependant on the baggage count for each one, weights being from 450kgs to 750 kgs the latter rare. Most will take between 40 - 60 bags and there is a full reconciliation of all bags and there bin location. Aircarft loading put simply uses the dry weight which includes a/c empty plus crew / catering and bags; same again plus fuel ; and then all plus punters / baggage / cargo. Key points after this are predicted departure weight after fuel burn / landing weight after journey fuel burn.