The basic events are "a particle moves from one place to another place" and "a particle is emitted or absorbed", and the amplitude or these events is "e" to the power of "i" times the "action" for this event

Is that like the idea that a particle being in a certain position has an amplitude? It doesn't. The universe does. It's just that if you pretended that a decohered particle was its own universe, you'd get the same results from much simpler calculations.

This does explain why physicists tend to write amplitude as a complex number. I've wondered that for a while.