The simplest way to understand how AI engines decide what to cite is to imagine a careful, well read research assistant tasked with answering a specific question. The assistant has access to thousands of sources but can only quote one or two in the answer. They will choose the source that is most likely to be accurate, authoritative, easy to quote, and trusted.
That mental model captures most of what matters. The signals AI engines use are the digital equivalent of how a human would judge a source.