If an indexing robot knows about a document, it may decide to parse it, and insert it into its database. How this is done depends on the robot. Some robots index the HTML Titles, or the first few paragraphs, or parse the entire HTML and index all words, with weightings depending on HTML constructs. Some parse the META tag, or other special hidden tags. You guessed it-it depends on the service.
Most services have a link to a URL submission form on their search page.
Fortunately, you don't have to submit your URL to every service by hand. You can check your server logs for sites that retrieve documents, especially in a short time. I've been visited by a robot. The whole idea is they are automatic, you don't need to do anything. If you think you have discovered a new robot, try to find its name on an active robots list.