By testing... do you think that doctors just create medicine at random, following recipes and never testing their effectivity, just trusting Big Pharma?

There are many laboratories in every country dedicated to the research of diseases. In fact, major universities have disease research centers as well, and there is a whole branch of science called Biomedical Engineering (https://en.wikipedia.org/wiki/Biomedical_engineering).

These laboratories are filled with people dedicated to this field. Many of them are passionate about what they do.