Is there any documentation on the primitive blocks?

Most of the primitives aren't self-explanatory.

If there is no documentation, is there a way to see the source code?

Great Question. There is documentation manual. Here is the link: SnapManual.pdf (berkeley.edu).

The code is also public. Here is a link for that as well: Snap! Build Your Own Blocks (berkeley.edu)

Also, there is help available (right-click the block) for most of them, all but the newest and the oldest. :~)

Do you think of untitled script pic - 2022-06-24T022510.157 ?
It's an internal interface to whitelisted Javascript functions that only optional libraries should use.
Menu :page_facing_up: > Libraries...
Not intended to be used standalone, but wrapped as a part of the library's custom blocks.
Documented by the comments in the "extensions.js"

what older blocks dont have help screens?

I think some that we inherited from Scratch.