In the fantasy culture it's not uncommon to find Giant spiders with the upper body of a human.

There are many variants as the spider body having only 4 legs and the upper human body have 2 additional arms or scorpion-like giant stings attached on the shoulders while some other variants even include ant-mimic spiders.

For my question I will use what to me seems the most classic option -

On a planet with conditions similar to earth's, why and how would this thing evolve? Would it be capable of producing and using its own silk as in the image above, and if so, how much silk to be more precise? If this thing is anyway realistic, can it walk on walls?

Bonus if you can describe its anatomy with precision, like if it's more convenient to give birth or lay eggs, or breath with the human part or the book lungs, and things like that.

This question is part of the Anatomically Correct Series.