Indexed, not submitted in sitemap

The “Indexed, not submitted in sitemap” status indicates that Google's indexed URLs that aren't included in your XML sitemap

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?

Those pages are in their index. They are missing in your sitemap. So those are OK. Generally the problem would be if you had pages Google doesn't know about. Sitemap is a way to tell.

But 19M pages? Seems that would be way off crawl budget for your website, and I am wondering what could potentially be on those 19M pages... I don't think we are talking unique content that does make any kind of sense.

Hello Try to put unique and original urls in the sitemap If some urls are not in the sitemap, it will be indexed again. But keep in mind that if these urls are of poor quality over time, Google will not index them. And most of these types of urls are created automatically or are site filters Note: should be further investigated and what type of urls you have. Thankful

When you submit a site map it indexed all the pages of your website but it will not index the pages which are not allowed to index e.g they are disallow in robot.txt file .