Is this the idea that our observable universe is actually inside of a larger black hole?

That's right

I've never heard of this. Any suggestions on where to read about these theories?

I think you just read everything there is to read about this. To my understanding, the idea that our universe is actually inside a black hole is not a “theory” in the sense that, say, relativity is a theory. It’s pure conjecture, and essentially not falsifiable. It’s more akin to the “theory” that our universe is a computer simulation.

So what happens to stuff falling into the outside black hole?

you probably still hit what looks like a singularity from your perspective and get spagettified

Wouldn't that be additional mass appearing from nothing from the perspective of this universe

Maybe this could be dark energy?

WARNING: This comment contains zero science. I didn't even Google it.

It’s the most intuitive explanation I’ve encountered for why we don’t see other life. (They might be inside of black holes.)

This is not intuitive to me. Why would they be inside black holes and not outside them?