Hard to explain (in part because I’m not a scientist) but it isn’t when you look at it, it’s when the photon interacts with something. I’m gonna do my best, and if I’m hard to follow, that’s because I suck at writing

Before interacting with something, a photon acts like a wave, kind of like a wave in water, or a sound wave. The wave goes through both slits at the same time, and causes an interference pattern on the other side of it. The pattern of stripes is caused by destructive interference—basically, waves have peaks and valleys. In places where two waves meet, their magnitude is added together. That is, where the peaks of those waves meet, the peak gets higher. Where the valleys meet, they get lower. Where a peak meets a valley, they cancel each other out. The empty parts on the detector are where peaks met valleys, and there was no measurable wave in those parts.

When a photon interacts with something, it collapses from a wave to a particle. I’ve seen it compared to a speck of dust in a raindrop. Before that raindrop hits the ground, you know that the speck of dust is somewhere in the drop, but not where it is in the drop. When it hits the ground, the speck can only end up in one spot. When the rain drop splatters, the speck of dust is more likely to be found in areas where more of the water went. That’s all the dots on the detector.

If the photon interacts with something at the slits, like a polarizing filter, it collapses before an interference pattern is able to form. No interference pattern means it ends up interacting with the detector in one of the two areas.