Advice on GEO courses / useful links?

Hi there, looking to dive deeper into Generative Engine Optimization but barely find any useful info, except the very basics. If I ask the same question using different services: Chat GPT, Merlyn, You and etc. they all give different websites even though I set to use "GPT4 model" and always add to forget /not to count on all my previous searches and act like I'm in incognito mode/first time user.. So there is definitely something I don't understand and want to learn more. Example: "Show me 10 best *** companies in **** area. " All the AI services I used give me completely different lists with about 2-3 repeated websites. So I'm really curious how those 2-3 websites show up in all the different AI searches... Thanks.