Search engine indexing
Indexing is the step where a search engine stores a page it has crawled so that the page can be returned in results — which is a separate decision from having crawled it.
Three things happen in order and are often confused: discovery (the engine learns a URL exists), crawling (it fetches the page) and indexing (it decides the page is worth keeping). A URL can be crawled and then not indexed, which is the most common and most confusing outcome.
The usual reasons for a crawled page not being indexed are that it duplicates another page closely enough to be folded into it, that it has too little of its own to say, or that nothing links to it so the engine sees no reason to spend the storage.
Submitting a URL — through a sitemap, Google's Indexing API or IndexNow — helps with discovery only. It brings the crawl forward; it does not vote on the decision that follows.