quantum mechanics - The relationship between angular and linear momentum

Why is orbital angular momentum not 0 when spin and linear momentum are not collinear?

Why can it be 0 when spin and linear momentum are parallel?

Like in the example of a scalar field at rest decaying into an electron and a positron via interaction $\frac{1}{2}g\phi \bar{\Psi}\Psi$

If spin is along x axis and three-momenta is along z axis, decay rate will be 0 if two spins are opposite because orbital momentum is not 0 after decaying.