This YT video (scrolled to 0:45) shows the process of fabricating a pole mount transformer. At 0:45 the following is shown: there's a wooden brick used to hold everything together, it's covered in cardboard and then the person starts winding wide thick aluminum tape onto this brick. Aluminum tape is as wide as the brick, so the brick is effectively wrapped into multiple layers of that tape. Adjacent layers are separated with thick insulating paper.
The core is fabricated much later, so it's not the core.
What's that tape for? It's thick so I'd guess it's the secondary winding (low voltage, high current, lots of heat) but then why is it closest to the transformer center where it's harder to get it cooled? Also I've disassembled a number of different transformers so far and I've never seen wide tape being used as a winding wire so I'm not at all sure.
What is this tape? Is it the secondary winding? Why is it closest to the transformer center?