A supermarket floor plan is an engineered environment built around commercial constraints. Those constraints happen to produce reliable effects on what shoppers buy.
Perishables sit at the perimeter for practical reasons
Produce, meat, dairy and bakery need refrigeration, drainage, water and frequent restocking from the back of the store, all of which are easiest to run along outer walls.
The advice to shop the perimeter is a byproduct of that plumbing rather than a nutritional principle, and it is imperfect as a rule since plenty of perimeter items are heavily processed.
It works reasonably well as a heuristic anyway, because the plumbing requirements correlate with foods that spoil, and foods that spoil tend to be less processed.
Staples are placed to maximize traffic
Milk, eggs and bread are bought frequently and by nearly everyone, so they are positioned to require walking past a great deal of other merchandise.
The route matters because unplanned purchases rise with exposure, and exposure is a function of distance walked and shelves passed.
A shopper who enters for three items and leaves with nine has not failed at discipline so much as walked through a layout designed to produce exactly that.
Shelf height is sold, not assigned
Placement at eye level and at the end of an aisle is commercially valuable, and in many American chains manufacturers pay for it through slotting arrangements.
The consequence is that prominence reflects a supplier's marketing budget rather than a store's assessment of quality or value.
Lower shelves frequently hold store brands and less-advertised products, which is why looking down is often the cheapest single change a shopper can make.
The checkout lane is a separate design problem
The queue is the one place where a shopper is stationary, mildly bored and already committed to a purchase, which makes it unusually effective retail space.
Items placed there are small, inexpensive and require no deliberation, because the decision window is short and the shopper is not comparing anything.
Stores that offer a lane without such displays are responding to a request from shoppers who found the placement effective enough to want it removed.
What actually counters the design
A written list works less because it improves memory and more because it converts each additional item into a decision rather than a default.
Shopping after eating and shopping without small children both reduce unplanned purchases, for reasons that are about attention and negotiation rather than willpower.
Online ordering removes the layout entirely and replaces it with a different set of nudges, so the influence is relocated rather than eliminated.