Light (in fact everything) is a wave, with some traditional particle properties added in. It's relatively easy to wrap your head around the weirdness from that point of view. It's almost impossible to make sense of it from a "particle with wave properties" view.

It's also worth noting that it is not observation, but measurement that matters. All observation is measurement but not all measurements are observations.

Basically, to measure something, you need to hit it with something else. Using a particle analogy (since the wave version is FAR less intuitive), imagine a pool ball, rolling down a table. You can only detect balls hitting the cushions. To measure where it is, in between, you need to roll additional balls across the table. In traditional physics, these balls can be thrown as lightly as you like, as accurately as you like. Unfortunately, the wave nature of the system imposes lower limits on this. When you throw a ball, it changes the ball it hits. To gain information, you end up damaging or destroying the system you are measuring.

In quantum mechanical terms, the wave function is collapsed. In fact, it's combined with the new particles you used to measure things.

In the original post. When you're not looking, the wave of the photon passes through both spits, it then interferes with itself. Only when it reaches the detector is it collapsed (by interacting with the atoms of the detector). When you try and measure which slit it went through, you introduce a new wave. This changes the shape of the original, and makes it appear like a particle.

This is quite a fun way of making yourself think in terms of waves. https://www.andreinc.net/2024/02/06/the-sinusoidal-tetris