Why is it "dans" mon canapé and not "sur" mon canapé ?

Prepositions are tricky in French, as they are in English. There are lots of situations where we use different ones. They usually use "DANS le canapé" and I guess you could remember it by reasoning that they sink in a bit and are enclosed in the comfy cushions. Similarly they would say "allez DANS la cuisine" where we would say "go TO the kitchen", or "je suis DANS le train" when we say "I am ON the train". Using prepositions is just a matter of constant listening and practice.

Qu’est-ce que c’est « un plaid »? Est-ce une couverture ?