I'd box it in and fill it with hydraulic cement

Do you only care about the moisture barrier?

A lot of suggestions are going to do what you've mentioned, and try to:

Make it structural.

Tie it into the existing foundation.

Prevent water intrusion

(in that order)

Moisture barrier alone? Get a piece of dimpleboard on the opposite side, possibly with some cementboard backing it to give it some strength.

Moisture and vapor? Peel and stick on the cement board, then as above.

Properly fixing it? Check to see if they did something fun with a sill plate and the rubble you're seeing. Clear out the rubble.

Drill into both sides of the gap about 6" and use simpson set-xp to install 4-5 pieces of #4 rod. (fitting the rod in both sides vs wiring 2 pieces that are in one side apiece together in order to develop the connection properly is above my paygrade)

Get plywood on both sides of the gap, and secure it to the foundation. (do more than just this, but I don't know forms well enough to tell you specifics) Put a bonding agent on the existing concrete to prevent a cold joint. Talk to the staffer at ( Whitecap/your concrete supplier ) and fill with the self-consolidating concrete or no-shrink grout they recommend.

Wait a week or so (mist the top daily with cool water) before you pull the forms and see how you did.

Edit: whoever did your foundation should get a good kicking for not doing their job properly, ditto the inspector for the city, and whoever inspected your home at time of purchase.

It LOOKS like they framed around it, so there's a chance this is only a cosmetic problem, but that needs an engineer to say for certain.

Thank you! That’s a great response. I’m going to check for a sill and see how stuck those rocks are.. I really like the suggestion of a bonding agent to create a moisture barrier. I’m not super concerned with the structural strength of it (but maybe I should be?).

The bonding agent won't do squat as a moisture barrier; it's a form of glue. It's only there to make sure the bond between old concrete and new is sound.

All told, this looks bad but is probably ok.

That said, I am a person on Reddit. If you want sureity, hire a structural engineer who'll come out and let you know what's going on. The few hundred it'll cost will pay for itself if you ever sell the house and the buyers go "what's this?"

This has gotta be between some additions or something, right? I can't wrap my head around what's been done here.

Really, I'd take a good look at the whole structure and try to see if that "weird gap" isn't there for a good reason. Sometimes things like that are designed in to allow for drainage or independent movement. I've made the mistake in the past of "fixing" things that didn't look right to me, and then later finding out exactly why they were the way they were.

Be careful. Not everything is meant to be sealed. For instance, people with rubble stone foundations (not hat you have one) always post on here about sealing foundation walls which in some instances could spell disaster. In certain applications you want moisture (not flowing water obviously) to be able to pass through so that it doesn’t build up pressure behind the wall. Also, with something like you have pictured here, that may have been done that way to allow flexibility for shifting. I wouldn’t just go about it Willy Nilly. Look around and see what the purpose of that might have been. There may be no purpose and it may just be sloppiness, but bottom line is, don’t assume it’s wrong the way that it is. The dirt looks a little darker at the bottom of the photo there. Is that moisture?

I was wondering about that.. lt wouldn't drain anything out of the basement, based on its position. It's not between 2 structures (is is part of an addition, but that was done at once). Maybe there is an air gap necessary when they put down the vapor barrier and concrete over the bedrock? Perhaps to let air in so that water underneath can drain away?
[edit: grammar]

Honestly, everything else about the house is very well done so it does seem odd that they would get that sloppy here. There is some rubble and a bit of moisture in the photo. It's possible that rodents dug the rubble out but the moisture is from a leaking hot water heater (this has distracted my DIY energy over the Christmas break [I actually made a hilarious "lazy river" with vapor barrier that flows into a bin and an auto-siphon to drain it outside so we could have showers and dishwasher over the holiday]. All to say that moisture is coming from a know source.

I'm coming around to the "this is supposed to be here" possibility. What sub would you post to get folks thoughts on what this might be for?