The difference is that it's something which already necessarily happens in quantum systems. Say you have two isolated particles A and B, and A is in a superpos

The difference is that it's something which already necessarily happens in quantum systems. Say you have two isolated particles A and B, and A is in a superposition between eigenstates 0 and 1. If those two particles interact, we say that they have formed a quantum system in a superposition between "A is in state 0 and B interacted with A in state 0" and "A is in state 1 and B interacted with state 1", right? MWI "simply" lets the same happen with observers. You have a particle in a superposition between states 0 and 1. You observe (i.e interact with) the particle, which puts the "you + particle" system in a superposition between "the particle was in state 0 and you observed the particle in state 0" and "the particle was in state 1 and you observed the particle in state 1". There is no wave function collapse mechanism, what we describe as "wave function collapse" is nothing more than the particles in the macro world becoming entangled with the particle we're testing.