How would somebody write their own search engine? I'm imagining some access patterns like this: If I know what site I want to get an answer from (youtube), i

How would somebody write their own search engine? I'm imagining some access patterns like this: If I know what site I want to get an answer from (youtube), i can just download lots of text from youtube and search those text files for the string im looking for, making a map of files that match that string (or youtube could expose an API for me to use of course) if i dont know what site i want an answer from, well this becomes harder, and presumably if i dont have the space to store a text only copy of the internet for me to grep through, then a 3rd party web copy (such as wikipedia) is probably a good starting point???