Just under the soil there’s a white chalky looking substance. What is it, please?

Question

On closer inspection it looks like threads of white compacted together.

Answer 1

I suspect it is fungal mycelium growing on a layer of organic matter.
https://en.wikipedia.org/wiki/Mycelium

But there is a chance it is a natural high calcium chalky substance, or layer of spilled or dumped cement spoils or paint from a building project.