There's no one model. How your immune cells pick their targets, or how your cells know which side is left or right during development. Neither is anything like how neurons work, or blood ion homeostasis is acieved. There are also (I believe) transistor like elements in metalloproteins that gate voltages in elecroactive systems like nitrogen fixation or photosynthesis (though that process is markedly dumber than the immune system).

Evolution is opportunistic and will reach for whatever computational method might be available and easy to fall into.