Many different books and novels contain creatures that have human upper body. But have the legs and thorax of spider. They are usually called by different names but they're sort of like centaurs but instead of a cross between a human and a horse they're a cross between a human and a spider. What biology would this creature need in order to exist without the help of magic?
So brushing away the problems with making a spider big enough to be "half human": What kind of world changes are necessary to make giant spiders feasible?
And get the "half human" effect by simply having a differently shaped cephalothorax, where the head and thorax are fused like in a normal spiders, except that the "head" is shaped to resemble a human upper body (in low light).
The pedipalps could act like the "arms" for instance.
Without magic, I would use evolution. Specifically create it in a world that would foster a creature like that and allow it to evolve. Appearance and biology would be more realistic. Then transfer the creature to your world. It wouldn't be the classic spiderling, but more believable than a lab created one (imo)
If lab or similar is the only option and relative realism is a focus, I would go with another creature; biology would seem trivial in that context
I already addressed some of this in this question here:
All you need to do is take the process there and apply it to spiders and then extend it. If it starts gathering food from trees then over time the front section of the creature would rise up and the front pincers or maybe even smaller limbs from around the mouth becoming adapted for fine manipulation rather than combat. Over time it could create what seemed like a humanoid body at the front, a process which would accelerate as intelligence increases and tool usage begins giving a real advantage to fine manipulation ability.