Too Long; Didn't Read
PageSpeed Insights and Core Web Vitals are testing a lot of different things, and a lot of stuff can be broken or require optimizations. Going through each possible problem and providing a fix for each use-case would probably add another 100 pages of text and code examples, and won’t be enough for all possible scenarios. Most issues which come up through the Core Web Vitals and PageSpeed Insights checks are technical and require a developer or technical solution to be solved. But understanding what the issue is and where to look for an answer is the first step to improvement.