What is seen in the bottom frame on the right , is a probability distribution: how probable is it to find an electron at the (x,y) of the screen. On the top left you see an invidual electron , whole, leaving a footprint on the screen consistent with a classical particle footprint. There is no spread of energy in space, as would be the case of the electron were really a wave, like water waves.
What is waving is the probability distribution, which reproduces a classical wave interference pattern.
The electron is a quantum mechanical entity/particle and the experiment is a scattering experiment "electron" scattering off "two slits a specific width and specific distance apart". The quantum mechanical solution of the problem is a wave function, $Ψ$, which in the theory of quantum mechanics will predict the probability distribution as $Ψ^*Ψ$.
So the electron passes as a whole from either slit with a given probability finally measured in the last frame.
Observing in physics means detecting interactions, not "looking" or "imagining".
Trying to check which slit the electron goes through changes the boundary conditions of the problem, because of the detectors introduced. It is a different scattering experiment has different $Ψ$ solutions which destroy the probability pattern seen in the last frame. This has been explored here
Overall, the results suggest that the type of scattering an electron undergoes determines the mark it leaves on the back wall, and that a detector at one of the slits can change the type of scattering. The physicists concluded that, while elastically scattered electrons can cause an interference pattern, the inelastically scattered electrons do not contribute to the interference process