Skeletons created them as disposable guards, after they realized they couldn't manufacture themselves anymore. As for combining DNA from locals, look no further than bonedogs.

I do not get too caught up in the story myself, but I do seem to recall the Shek being made to "police" something.

They were replacements for the Hydraulic Knights IIRC, also to fight the rising tide of cannibals. They still kill skeletons even though they couldn't eat them. I can't remember the channel I saw it on, though I'm not keen on digging through 430 subscriptions to find it . U tubes category system leaves a lot to be desired. The source is accurate.

I stick with the theory on the wiki. It's based on hints in the game and much less fancy.

https://kenshi.fandom.com/wiki/Shek

As far as I'm aware they are descendants of the genetically modified humans known as "Enforcers"