The search is by depth of further moves, and “better” is a function of heuristics (explicit or learned) on the resulting board positions, because most of the time you can’t be sure a move will inevitably result in a win or a loss.
So any particular move evaluation might take more or less time before the algorithm gives up on it—or chooses it as the new winner. To throw a consistent amount of compute at each move, the simple thing to do is give the engine consistent amounts of time per move.
