I want my ghosts to just be semi-transparent sims but I couldn't figure out how to do it, but a new mod just came out, that for one of the interactions lets you project a semi-transparent version of your sim (obvs it's a super cool mod even apart from that and I'm def planning on downloading it) - but how do I find where that shader is and extract it, and how do I replace the regular ghost shaders with it?

I don't think this was achieved with standard shaders (at least not in the way they're utilized in CC), but rather a more advanced scripted effect.
Assuming you're talking about Xantak22's Imaginary Friend mod, this question would probably best be directed at them as we can't really have much insight on how other people's scripts work. Aside from asking Xantak, the MTS Create forums are often better suited for mod creation questions as well.