Taking a peek beneath the motherboard, we make a couple cool observations.

That massive copper heat pipe under the board is much beefier than the one in the S9—it looks more like the one we found in the Note9.

Meanwhile, we peel off an additional, multi-layer piece of thermal interface material from the board. All that copper makes a great, big, flat surface, for better thermal transfer—but it's soft metal, so you need this soft interface to fill in any gaps that might otherwise kill performance or overheat your phone.

This thin sticker also seems to provide some RF shielding, as there's a big hole in the can lid underneath—where we find a PMIC and a big pink thermal pad.

TL;DR: We surmise that fast charging + reverse wireless charging puts some serious thermal stress on the electronics in this system. Samsung has pulled out all the stops to cool it off.