Google's Knowledge Graph is the most famous; Wikidata is the most open; every major AI retriever now builds its own internal graph. These graphs underpin the 'people also ask' / 'related products' / 'similar brands' suggestions that AI assistants surface.
To be in a knowledge graph you have to be recognisable — correct Organization schema, sameAs links to open sources, and consistent mention across independent domains. Once you're in, your entity becomes a hub that other entities can link to.