Literature review research is an iterative process, so you’ll repeat these steps over and over until you have at least 25 sources.
You’ll do an initial search for preliminary literature. Those sources will help you locate seminal (foundational/landmark) and recently published literature (the middle circles). If your research question is appropriately focused, you’ll start to see the same authors and sources cited over and over again in your reading. That’s how you’ll know you’ve tapped into the major research on your topic.