You have found a very common mistakes in low-end (e.g. YouTube) explanations of YDSE. The electron always acts like a wave. When you have which-way information, the patter on the screen is two overlapping diffraction patterns:

$$ \psi_{inc}(\theta) = ||A_L(\theta)||^2 + ||A_R(\theta)||^2 $$

where the $A({\theta})$ are the amplitudes from each slit.

When you don't have which way information:

$$ \psi_{inc}(\theta) = (A_L(\theta) + A_R(\theta))^*(A_L(\theta) + A_R(\theta))$$

$$ \psi_{coh}(\theta) = ||A_L(\theta)||^2 + ||A_R(\theta)||^2 + [A_L(\theta)^*A_R(\theta) + A_R(\theta)^*A_L(\theta)]$$

where the term in square brackets represent interference.

Note that the wide diffraction terms seen in the incoherent case are identical in the coherent case.

It's always a wave formed by summing the amplitudes coherently over all allowed paths. In the case of single slit diffraction, that is an integral over the width of the slit, and that looks just the definition of a Fourier transform of the aperture profile.

You have found a very common mistakes in low-end (e.g. YouTube) explanations of YDSE. The electron always acts like a wave. When you have which-way information, the pattern on the screen is two overlapping diffraction patterns:

$$ \psi_{inc}(\theta) = ||A_L(\theta)||^2 + ||A_R(\theta)||^2 $$

where the $A({\theta})$ are the amplitudes from each slit.

When you don't have which way information:

$$ \psi_{inc}(\theta) = (A_L(\theta) + A_R(\theta))^*(A_L(\theta) + A_R(\theta))$$

$$ \psi_{coh}(\theta) = ||A_L(\theta)||^2 + ||A_R(\theta)||^2 + [A_L(\theta)^*A_R(\theta) + A_R(\theta)^*A_L(\theta)]$$

where the term in square brackets represents interference.

Note that the wide diffraction terms seen in the incoherent case are identical in the coherent case.

It's always a wave formed by summing the amplitudes coherently over all allowed paths. In the case of single slit diffraction, that is an integral over the width of the slit, and that looks just the definition of a Fourier transform of the aperture profile.