I'm wondering what are the considerations for having multiple detectors instead of a single large one! Does anybody have an idea? is it just because it's easier?

The primary detector is the wide-field instrument, which, believe it or not, takes pictures with a wide field of view. The secondary detector is the coronagraph instrument, which uses a bunch of funky filters to block out the light from the star to see the halo around it.

Edit: These are the detectors, which are the cameras. There is only one mirror assembly (with two mirrors) that focuses light into these instruments.

Cool!

I wonder how different those are to real simulations! Do they treat electrons as point-like, or it's all computation on the fields?

The simulation does calculations on the fields only, so it keeps track of the average electron and hole density at each point in space. There is, however, a one-to-one correspondence between the behavior of the fields and the motion of individual particles, which is what makes these animations possible. What I mean by this is the diffusion equation is satisfied by the probability density of a particle undergoing a random walk. So given an electron density that obeys the diffusion-drift equations we can make dots undergo a random walk with drift that turns out to match the given density function (the result is the second set of animations).

Do the electrons in this visualization also repel each other?

The electrons are repelled or attracted by the electric field, which is in turn determined by the average charge density. So two electrons in the animation don't directly repel each other, but the electrons do influence each other through the field.