Nobody is being 'told'. But every booking system I've ever used, whether it's airline-direct or an agreggator shows aircraft model.

Flightaware and others show it too if you just search for the flight number

BA tell you the model of plane that's scheduled when booking.

I've actually selected a different flight a few hours after another so I would be on an A380 vs 747 because two hours extra waiting to come home is worth it for the comfort on a longer flight, and feeling more refreshed the other end, especially with a long drive back from the airport waiting for me.

Depends on when you book and what fleet they have. The exact plane (individual) isn't assigned to your flight by the scheduling software at the booking date, if you book months in advance. It's a guess in that case, and it can change up to hours before the flight. So there is nothing that guarantees that the model specified on your booking will actually be used.

I can always see the type of aircraft when booking a flight with KLM. I don't know if it's typical though.