This may be a dumb question, but how do these engines get better when they're already amazing? Is it simply that they look further down the tree of possibilities?
When you're amazing, you understand what you're doing and where to improve. For very complex systems, the improvements will be in confined areas (not rewriting everything but noticing a bug or reading about another project which found that some other hyperparameter works better). Or even if you have no idea, the simplest way would be making random tweaks and seeing which ones are advantageous