I don't think there is much info about them except that the Old Ones created them, and they presumably work magically or at least are controlled magically.
I believe they work like this:
- Spawning pools contain something like "Bathybius haeckelii"
https://en.wikipedia.org/wiki/Bathybius_haeckelii (only it is real), so some kind of primordial matter,
https://en.wikipedia.org/wiki/Primordial_soup
- given enough time, life spontaneously develops from that matter
- Since that would take WAY too long (millenia) and produce something very simple, and even worse, random, the Old Ones created either machinery or magic (Clarke's Law #3 says "Any sufficiently advanced technology is indistinguishable from magic") below or around it that manipulates the matter. That manipulation uses the primordial matter to form genes, and cells are built. Within the Spawning pool there is a rapid development of individual lifeforms, which results in adult Lizardmen being born from it.
- Spawning pools need tending, which probably means putting material in there, and also controlling the mixture to a certain degree, along with the temperature, the pH-Value, and other parameters. That is done by specially trained Skinks. It is also possbile that the Skinks really only put all the stuff in there and the machinery/magic does all the adjustments but I like the idea that the frequency of spawns and other parameters for the spawning pools can be adjusted a little bit by the Skinks or Slann.
Such as increasing the temperature a bit to get smaller Skinks to ride beasts, or making more Kroxigor by lowering it a bit. (in real life Crocodiles instinctively adjust their eggs' temperature to control the sex of their offspring so that isn't too deep into the realm of science fiction or fantasy.)
- The Lizardmen don't fully understand them, and they fear breaking them so much that are not willing to make dangerous experiments with them. Everythign they know about them are minor tweaks that were discovered accidentally or after long and careful contemplation and discussion among Slann.
- Demons (especially Nurgle and Tzeentch) probably know how to corrupt them in order to corrupt them. So of course they are guarded as well as possible.