The testing difference is the part nobody explains well

I want to be careful here because this is the sort of topic where people overstate things fast, and I am not going to make a claim about safety that I cannot back up. But the piece I never see explained clearly is what testing actually involves and what it does and does not cover. Most of what I read is either an advert leaning on the word tested without saying anything, or a scare story with no documentation attached. Neither of those helps someone trying to make an ordinary decision, and both are apparently very effective, which is the frustrating part. So my question is whether there is a plain, boring, official explanation of what is required to be tested and what the results mean. Not marketing, not somebody assurance in a comment. I would rather read the dull version and understand it properly.