So my question is, how are the starting points for those Kangaroos created or selected?
Random, within some interval. For SOTA, these intervals are shown on the diagram on this page:
https://github.com/RetiredC/Kang-1Therefore the rest of your assumptions are incorrect, learn sources to see how it works.
When huge number of kangaroos is used basically you can assume that you use birthday paradox to find a collision.