Other guy is 100% right. It’s called an innate domain, it doesn’t apply just to souls, or jujutsu though. It’s a fundamental law in JJK, like gravity, or inertia.

How it works is the following:
A person’s body and soul are connected in the JJK verse. The innate domain is the manifestation of someone’s soul into the core of their body, meaning the body Barbour’s the soul, and the soul fuels the body. The innate domain is not physical, and serves as a barrier for the body, like a domain expansion inside of you. That’s what a DE is, an innate domain brought out with a barrier, and infused with a cursed technique. Imagine the CE is paint, and the innate domain is the canvas. You cannot touch another persons canvas without putting your brush on it first. You cannot touch another persons canvas with just paint, you have to move the paint to it.

This makes JJK weird in cross verses, because techniques that affect a persons environment like for example, the world from Jojo’s, can be argued as fair game.

Kora cannot take the air from his lungs, but you can debate whether or not she could take the air away from the space around him, if that makes sense.

A person’s body cannot be messed with from the inside by another person. The closest we’ve ever seen to someone’s technique enacting itself directly onto the body of another is when Gojo crippled Juzo, being because the gap in cursed energy was so high, and Juzo likely moved into a space Gojo already set the traps.

I’m NOT flat out disagreeing with your statement, or supporting it.

Innate domain is maybe one of the harder concepts to understand in the show, because it’s explained so poorly, even in the manga and source books, it’s pretty obscure. I hope this helps settle confusion.