E. Marten

You need to use WordPress SEO by Yoast (Props to him!)
Adds “More Sitemaps” to your WordPress SEO Settings. If you are using a second WordPress-Installation in a subdirectory, you can now easily add the second Sitemap into the main one.
If you are using a second WordPress-installation in a subdirectory and want it to be fully integrated it into your main site to prevend struggeling with two or more Sitemaps in your Webmastertools, 
you can easily integrate more sitemaps into your main one. (sitemap_index.xml) 
The Plugin adds your second and third (aso) sitemap into the main sitemap and tooks care about the modification date automatically!
Thanks to Joost de Valk!

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension

Adds "More Sitemaps" to your Wordpress SEO Settings. Include differen sitemaps to your sitemap_index.xml

Bill Minozzi

★★★★★
<blockquote>
This plugin Disable WordPress native default Sitemap automatic creation.
</blockquote>
The sitemap is created automatically by WordPress from version 5.5. 
This plugin just give you option to disable it. 
Very simple, not administrative panel available. No extra configuration required. Just install and active the plugin. That’s it. 
You can activate or deactivate it from your plugin’s page.
Features 
* Simple to use: just activate it. 
* Lightweight 
* Do the job with a click.
Why to use this plugin? 
The “Disable WP Sitemap” plugin can be useful in certain situations. 
The XML Sitemap helps to index your website in Google or any other search engine. 
If you already have customized and configured your present sitemap you don’t need to do this again for WP Default sitemap. 
Having multiple sitemaps in a single website may confuse the search engines. 
It is easy to use. Just install and activate it. No extra configuration required.
This plugin requires the free Site-Checkup plugin.

Disable Sitemap (Deactivate Sitemap)

Disable WordPress native default Sitemaps automatic creation from version 5.5.

IsSiY (Include second Sitemap into Yoast) WordPress SEO Extension vs Disable Sitemap (Deactivate Sitemap)