Hello,
Thanks for contacting us and we regret the inconvenience caused.
Same as Rank Math, Yoast also generates the Sitemaps on runtime and you would not find a physical sitemap using FTP.
You can check this thread of the Yoast support forum on wordpress.org for a quick reference:
https://wordpress.org/support/topic/cant-access-sitemap-2/#post-12351246
Unfortunately, it is not possible to generate physical sitemaps with the plugin, with that being said, it does not matter if you use a physical sitemap or a virtual one, as Google can easily read dynamic sitemaps.
Also is there a difference between editing the robots.txt file inside RankMath settings in wordpress vs editing it inside the FTP?
Since the robots.txt file is also generated on runtime, you’ll be able to edit the robots.txt file with Rank Math only when there is no physical robots.txt file present on your website’s root directory.
Again, it does not matter if you use a physical robots.txt file or a virtual one, since the file is created as per the norms of Google, they can be easily crawled.
Hope that helps.
Thank you.