One SEO professional asked John Mueller in a hangout about how sitemaps are crawled.
Their main question is: when Googlebot crawls the sitemap in the server address, does it crawl the submitted sitemaps in the GSC backend, or does it go directly to the site owner’s server address to crawl these sitemaps?
John explained that they don’t crawl the sitemaps all at the same time. So it’s not that there is a specific order to them. It’s more that for individual sitemap files, Google tries to figure out how often they change and how often they give Google useful information.
Based on this, for individual files, Google will crawl them at different rates.
It’s possible that what may happen is one sitemap in their Google Search Console account is crawled very frequently, but also one that they may submit directly is crawled frequently, and maybe another sitemap in Search Console is crawled very infrequently.
It doesn’t depend on where you’re submitted. It really depends more on the sitemap file itself and what value it shows Google.
This happens at approximately the 11:42 mark in the video.