How do I keep this from happening

I don’t know how to explain it, but it’s like a paper-thin sheet that just stays there after a union.

this is the union:

Yes, scale out the pink negated parts further out, they arent further than the edge of the main piece you are carving out.

Well thats kinda un do-able, since I’m trying to make the limbs from humanoids.
Surely there’s a way for me to get the humanoid limb mesh, right?

Why is it not doable? Grab the pink piece and hit scale and pull it out further, or even grab the move tool and bring it out a stud or two past the edge of your big piece… then no paper thin edge is left.

The angle on the part will not be what I want it to, but I can try something with wedges and parts.

The angle doesnt need to change… just scale the part slightly larger… angle stays the same.

any ideas how to fix this?

Use corner_wedge part and scale it.

I ended up using cylinders and spheres, it looks much better than what I had before.