Isn’t private credit defined in part as “lending by non-banks”?

Like, when a bank originates a mortgage, that mortgage gets traded, much like private debts don’t.

I don't know a lot about finance. What is the definition/significance of "firm" in this context (if that's not a complicated question)?

That's not correctly stated. "Private Credit" is defined as non-bank lending. Banks are doing "public" lending in the sense of being regulated. Private lending is any sort of financial instrument issued outside of those guard rails.

It's generally felt to be risky and volatile, but useful. Basically, it's never illegal just to hand your friend $20 even if the government isn't watching over the process to make sure you don't get scammed. This is the same thing at scale.