As to whether buying something from a shop and heating it in the oven counts as "cooking" ...
I'd say that if something ends up hotter, or different, than when you started ... then you've cooked it.
A ready meal ... buy it, heat it in the oven, eat it.
A leg of lamb ... buy it, heat it in the oven, eat it.
There's no real difference.
If you were being really picky, you might argue that one doesn't cook a sandwich.
Although, if your criterion is the fact that a sandwich doesn't get hot, then you aren't cooking monkfish ceviché. But you are cooking a bacon sandwich.