Basically the sensor is interacting with the quantum particle by measuring where it is before the slit. This collapses the wave function of the particle, causing it to appear at one point in space. Since the particle is collapsed to a point before the slit, it travels through only one of the slits and impacts the screen. Since it is just a single point particle now, there is no wave to interfere with the particle and create multi-line wave pattern, so we just see two straight lines on the screen that match up with the slits.

The sensor performing the measurement is the observer in this case. No living creature is needed to observe the particle to make it collapse. It’s simply just, quantum particles are just wave functions up until the point that they have to collapse to a particle because it has interacted with something (a screen or a sensor or anything). That is about the limit of my understanding at least