Tesco is probably a bad example because it seems to have been run poorly. It actually has assets which are at about 12 billion, so less than the debt on the books. Their debt seems to be about 3-5 times that the normal, giant, supermarket. As an example, Aldi has basically no debt and Sainsbury's has about 4 billion or something. Just points to a poorly run business imo.
The margin is more than sufficient. It generates 65 billion in revenue. Obviously overheads, costs, and all of that leads to a fraction of that being net income. But other supermarkets are getting along just fine with the same if not similar margins. And making a killing, too.