No, a spider is not an insect. Spiders have a slightly different structure :) Unfortunately, I can’t say for sure what the differences are (other than the difference in legs), but spiders are closer to scorpions than to insects; I can say that with 100% certainty ;) (and you can trust me – I passed biology in 9th grade, and I had to dissect the entire textbook...)
*added after diligently flipping through the textbook*
Arachnids have 4 pairs of walking legs, and their body is divided into a cephalothorax and an abdomen, or not divided at all. Insects have 3 pairs, and their body is clearly divided into a head, thorax, and abdomen.

I’m here to educate you!))))