In Tribodar Permaculture, Portugal, the amazing people there have experimented with all sorts of different types of alternative sustainable ways to build structures. One of them involved the use of recycled fridges. Placed one next to another, some facing inside the space and some facing outside, they form the walls of the shower area and parts of the walls of the small house.
This innovative idea ticks a lot of sustainability boxes and has also another benefit: these walls are basically storage space. There is a downside to that though, opening the fridges during the winter to store things inside creates a thermal bridge.
So the best way to use fridges as a building component for walls would be either to build internal walls, or build the external walls of a space that doesn’t need thermal insulation. Storage rooms, workshops, garages, you name it.