I've noticed this in console since I added an additional 18 million+ pages on my site without submitting sitemaps for the new links.

Google has only indexed maybe 400,000 of the pages which are all "Indexed, not submitted in sitemap", but I am still wondering would it make any difference if I made a sitemap index and created the 380 sitemaps of 50,000 for the index that is 19 million pages?