Individual contributors is just a fancy name for a software engineer without direct reports. Aka you'd stop being a manager and just write code (for the most part).

Anyone non-tech I know would just call it "labor", as opposed to management. The people who mostly do the things that make the stuff, rather than mostly telling others to do the things to make the stuff.

Again, people'd just say "labor" in other contexts. Or maybe "workers". I'm not sure where "IC" came from but I've never seen it outside of tech, and that, mostly online.

I think “labor” is associated with repetitive or at least well characterized jobs

In tech, any two given workers are much less likely to be doing the same thing. So they are clearly not just laboring but contributing at some creative and self-management level as well

Non-managers, they aren't responsible for anyone but themselves. I know nothing of Meta's specific titles but I'd expect that e.g. the first level of full time engineering manager would convert to a full time senior or staff engineer.