what's the difference between stdlib and stdlib-jdk8?

in what regard? stdlib-jdk8 is the thing that let's you interop with jvm code

I believe it has some extra functions relating to Java 8 things, IIRC there's something related to autoclosable there.

I mean, stdlib does too right? I guess if I'm running on jdk 11 I'm not sure what I'm supposed to be using, the docs aren't immediately clear why I'd pick one... I'd presume the jdk6? one fills some gaps?

If I get it right, stdlib-jdk7 and stdlib-jdk8 provide additional functionality in the case you target Java 7 and 8 platforms.