Don't understand minimax algorithm in terms of chess
In minimax you fully explore the game tree. So the answer to "which piece" is all of them. All possible moves for all movable pieces. And then it selects the minimum or maximum (depending on the depth) out of those options.

Obviously this isn't feasible which is why we never apply raw minimax to chess. It is always with other optimizations such as alpha-beta, and stopping after a certain depth and using a heuristic evaluation of that position instead of continuing.