Various mechanical contrivances have parts that attach by internal threads.

The LAW folding stock adapter is a great example, but I've even seen this design on other things, including motorcycle carburetors.

The LAW adapter has a sort of reverse castle nut that screws into the receiver shock tower, but still allows the extension on the bolt carrier to pass rearward.
The reverse castle nut takes a tool that fits into slots on the nut and you can then tighten it. The tool is simplicity itself: about 2mm thick, exactly as wide as the nut I'm describing, and with wings at the top
you can apply pressure to in order to tighten it.

What the hell is that tool called? Maybe it doesn't have a name.
I was helping a friend put together a carburetor and he had a smaller version. He called it a carburetor tool (because he didn't know the name it either).

Help a brother out here.......

IIRC it's actually a mossberg (I think, could be another brand) choke installation tool.

Adjustable pin spanner wrench?

"Internal spanner" is the best descriptive term for that tool.  But, like most things, there are going to be numerous sizes, differing number of pins/bars/lugs and shapes of the lugs.  Each manufacturer of the things are free to use whatever best suits their application. There is no universal accepted standard for most of them---unlike socket wrenches and their bolt heads.  That is why some of the things are adjustable.

What you are describing are various types of spanner wrenches:
https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1430