> What decision-making process led to the idea of injecting human urine into a frog in the first place?

Hormones are basically messages sent through an animal's body to signal some change should take place. It was discovered that there was a hormone called hCG produced by the human placenta that triggers "you're pregnant" changes in the body. hCG is also present in the urine.

So if you want to detect a hormone, the idea is you inject it into an animal and see if it triggers the relevant changes (since the changes are usually internal, you generally need to kill the animal to check). So you would look for an animal that responds somehow to the hCG hormone, inject urine into it, and check for the response. Mice and rabbits were first used, but it was eventually discovered that certain species of frog that are highly sensitive to hormonal changes made for much simpler and faster testing.