Hiding beneath the logic board, we find a teensy switch that makes the mouse click its click (no Taptic Engine just yet).

Luckily, it's held in only by the board above, and is a welcome relief after wading through the rest of this tar pit.

As a common failure part for computer mice, it's nice that Apple used a fairly standard and easily-sourced switch—although its replacement will require dealing with all of that glue (and soldering in the replacement switch).